I agree with Beatrice... if it was most other birds, you might have a chance, but if you do some reading on sonnies you will see one thing they are notorious for is being one person birds. They are awesome birds for single people who if only one person wants a bird, they truly are amazingly devoted and loyal to their chosen person.
I can share my personal experience... a Senegal courted my husband at the shelter for over a year. From the moment they met, Mali only wanted to hang out with my husband, but it meant giving up a space for a foster baby and another cage I would have to clean. My husband loves playing with the birds, will help with feeding and watering, but not one to jump in for cage cleaning

But, after a year of introducing Mali to adopters and Mali sending them away bleeding and only being good for Byron, the decision was made that it was pretty clear Mali was not gonna change his mind and Byron was his chosen person. Now up until that point in time, Mali had tolerated me and been fairly social, had nipped me a few times, but not the vicious hang on and see how flesh he could remove as he had done with other people. I joke he was only sucking up to me to get his ticket home. Mali has been in my home since Valentine's 2015. I am the person who cleans cages, prepares all the fresh foods, hands out treats, replaces toys, does the majority of feeding and I even take the little turd face out to hang out on playstands during the day when my husband is at work. So you would think that from February 2015 til today Mali and I would be friends... well heck no... Mali goes for flesh any chance he can, he refuses to step up on my hand (okay not true, he steps up but there is blood in involved and he does not let go). He will step up on a towel, but then does his best to get thru the towel to my flesh. He is stick trained for moving him from cage to playstand and such. He will let me scratch his head when he wants his head scratched, but if I'm doing food and water and turn my head for a second, he's going for flesh. He just nailed me the other night while I was adding nutriberries as a night snack. Through the bars, saw his chance and went for it. He has wing atrophy (was about 25 when he came home to us), so he cannot fly, but if he gets on the floor, the sucker will literally walk across a good 50 to 100 feet to track me down and try to attack me
So, if you have tried making friends with him and he only wants your girlfriend. Just accept it. Much like everything in life, you can't have everyone love you!!
Getting a new bird is not a big deal as long as the birds have their own spaces and your girlfriend makes sure the sennie knows he is still number one with her. From the moments of you saying he gets an attitude with your girlfriend, be prepared that if he sees her being affectionate with the new conure, he will express his jealousy. Mali gets very upset and jealous when my husband spends time with other birds and has given some pretty significant chomps to express his displeasure. But we are a home with multiple birds, I've posted before, its like having kids, make sure everyone gets attention, is loved and has their own space, then you can be a happy family!! But your worry about the Senegal being aggressive with the conure is a valid worry, so again, each having their own playstands or space is the simple answer.
I would also consider adopting an adult bird, but that's a personal opinion and going to a rescue and having a conure pick you as their person, will help to establish relationships!
Best of luck!!