Birds choose "their person" pretty quickly, and then that's it...There is rarely any rhyme or reason as to why they choose the person they do, a lot of the time they choose the person who wants nothing to do with them and they dislike the person who does everything for them...Don't try to "figure it out", as it just is what it is, and you aren't going to change the fact that he has chosen you as "his person"...It's not at all that he's "too bonded to you", you could totally ignore him 24/7 and you're still going to be "his person"...So you need to stop thinking that way, like you want him to be "less bonded to you", because that's not at all how this is going to work...
He doesn't "hate" your girlfriend...He's jealous of your girlfriend, and also, in addition to jealousy, he's actually trying to "protect" you from her...So every time she touches you or goes near you he's going to have to protect you...And when you're not around, then she isn't a "threat" to you, and he doesn't need to be jealous of her because she isn't touching you or "hurting" you...That's how your bird is thinking...
The idea here is NOT going to be ignoring your bird or keeping him in his cage all the time, or trying to make him "less-bonded to you", but rather making your girlfriend use "positive reinforcement" with him whenever she can, and then also having YOU take control of the situation and letting him know the way things are going to be...Let her be the good-guy all the time...She's the one that gives him all of his treats and does all of the good stuff all the time. And whenever you two are together and he tries to bite her, lunges at her, or even screams at her, or starts being aggressive towards her in any way, it's going to be up to you every single time to show him it's not going to fly, and he must stop...The most-effective way of doing this with "Velcro Birds" like Cockatoos, Conures, etc. is by using the "Shunning Method"...
The "Shunning Method" is quite simple and works very well, very quickly, because they absolutely hate being ignored by "their person" more than anything else in the world...However, you absolutely MUST do it every single time he becomes aggressive with your girlfriend for it to work. If you give-in, feel badly for him, or just don't do it every single time he is aggressive in any way to her, he will feel that he can get away with it and he'll start doing it again...If you do it every single time he's aggressive/lunges at/bites your girlfriend in your presence, he'll get the picture very quickly.
All the "Shunning Method" requires is this: Every single time your bird becomes aggressive towards your girlfriend in any way, whether it be a full bite or just a lunge towards her, or even a direct scream at her, you immediately have to take control of him, and immediately say the same phrase that you choose, something like "No Bites!" or "Bad Bird!"; whatever phrase you choose, it needs to be short and to the point, and you must say it firmly and loudly (don't ever yell at or scold him angrily though), and say it immediately upon any aggressive behavior towards your girlfriend...So as soon as he lunges/bites/screams at her, whatever it is that he does directly to her, you immediately say the phrase, such as "No Bites!" or "Bad Bird!", and then you immediately put him right down on the floor. They hate being on the floor because they are the lowest thing in the room, so they have no power or dominance at all over anything or anyone, and even worse is the fact that YOU, "his person" has put him down and are ignoring him. To your bird, this is the worst thing in the world... Then as soon as you put him down on the floor, you must immediately turn your back to him. Literally. You turn your back to him (and your girlfriend and anyone else in the room must do the same, he needs to be ignored completely by everyone). You turn your back to him, and then you must completely and totally ignore him for 5 minutes. No less, no more...Too short a time period and it means nothing to him, too long and he'll lose interest or become so frustrated that he misses the point...
While he's on the floor and you are totally ignoring him, you must not make eye contact with him, don't look at him at all, don't even acknowledge him being there, and don't say a single word to him or make any acknowledgement if he makes any sounds. Ignore all of his screaming/crying/whining noises, ignore him if he calls to you, just purposely pretend that he doesn't even exist for 5 minutes...If he walks around to your front to look at you, you turn your back to him. If he starts climbing up your leg or he flies to your shoulder, etc., DON'T SAY A WORD, just quickly remove him and put him right back down on the floor and turn your back to him again. Some people will actually walk out of the room, but I don't think this is nearly effective and I don't suggest doing this at all because #1) Him actually seeing you purposely ignoring him is much more impactful to him than you just leaving the room, and then also #2) If you leave the room he's in, chances are that he will start crying or screaming, and this usually continues for longer than 5 minutes. So if you leave the room and he starts screaming, and then the 5 minutes is up and he's still screaming, you can't go back to him because then you'll be rewarding his screaming, and with a Cockatoo this is a very horrible thing to do, lol...So this works best if you just stay in the same room he's in and continually keep your back to him, don't look at him, don't say a single word to him, put him back down silently if he flies/climbs back up to you, and once a full 5 minutes is up, then You don't just immediately go back to paying attention to him or immediately pick him up again!!! When the full 5 minutes is up, you simply "stop purposely ignoring him", meaning you stop keeping your back to him, you look at him again, etc. The best way I've found to come-out of the 5-minute "shunning" period is to simply turn around and face him again, and then simply go and sit down somewhere or just start doing things normally again, and allow him to come back to you on his own, and when he does only THEN do you start talking to him normally again, pick him up, etc., and act perfectly normally again, like nothing even happened.
****Don't be surprised if you do this the first time, you get through the 5 minutes, you then go and sit down and allow him to come over to you again, and you start talking to him and holding him again, and he immediately tries to bite your girlfriend again...It will take a few times for him to get the picture...What's very, very important is that if you do this, the 5 minutes is up and you go back to treating him normally again, and he just turns around and is aggressive to your girlfriend again, you immediately do the "Shunning Method" again, starting completely over, and you do it as many times in a row as you need to until finally he stops being aggressive towards her, it's his bedtime, whichever comes first. Then you'll need to continue to do this every single time he is aggressive with her from that point forward, until he gets it and is no longer being aggressive/biting/lunging/screaming at her, and he seemingly "accepts" her. It you don't do this every time he acts aggressively towards her, then HE WINS. So let's say you do it, the 5 minutes is over, he comes over to you and everything is normal again, and then he immediately screams/lunges at/bites your girlfriend again...So what do you do? You immediately say "Bad Bird!" firmly and loudly again, put him right back down on the floor, and turn your back to him for another 5 minutes of purposely ignoring that he even exists again...And then so-on...
I've seen this become successful in one evening of doing it maybe 4-5 times in a row with a Cockatoo, a Macaw, African Grey, and even with a Green Cheek Conure, as they are all so very intelligent and also so very needy and clingy with "their person" that this just devastates them at first, and then it eventually just pisses them off so badly that they just kind of give-in and say "Okay, I get it, I'll stop, she's okay now and I know she's not going to hurt you or take you away from me." But YOU must remember that this is all dependent on YOU being the one who takes immediate control of the situation and then totally depriving him of the one thing he loves the most in the world...YOU.
****And I'll say this one more time to you because it's extremely important that both you AND your girlfriend understand why he is doing this, and that you don't keep trying to figure out WHY he has chosen you and not her, or trying to make him "less-bonded" to you, that's a horrible idea to even attempt, because not only is it a lost-cause that usually just makes the problem much, much, much worse and does nothing to stop their protective/jealous behavior over you, but it also usually causes new issues to start, such as him getting so frustrated at you for purposely trying to always be more distant with him that they can often times actually start plucking themselves or even self-mutilating.