While I can't offer a lot in the way of helping you feel better about losing family members and friends, including Kizzy, as that will come in your own time, what I can tell you is that you cannot base your own happiness or saneness on what others say and do. Every person is an individual, every person has had different experiences, and yes, some people, a lot of people, are just nasty people who were never taught to feel empathy, compassion, or any type of love for other human beings. They simply say what they are thinking without ever considering how someone else might feel. They are incapable of putting themselves in the shoes of other people, usually due to the fact that that's how they were raised and they were never taught to respect others, or to put themselves in the shoes of others and take their feelings into consideration before opening their mouths or doing something stupid. That all being said, even though it hurts you, frustrates you, and makes you feel worse than you did to begin with, you have to ask yourself "Why does what this person thinks matter to me anyway?"
As far as medical practitioners go, I get it completely. It's like they don't bother to screen people they hire for compassion, that seems to be pretty universal in this country. I developed endometrial cancer, have had several major operations, treatments, etc., and I can speak from experience when I say that quite a few receptionists, techs, nurses, doctors, pharmacists, etc. act like robots, don't listen to what you say, and certainly don't really care. Try telling a male doctor in his 50's about how badly endometriosis and ovarian cysts hurt, to the point that you can't move, let alone work a 12 hour day. That will usually get you called a drug addict looking for narcotics. And I shouldn't make this specific to men, because I had one female gynecologist a long time ago that was the Antichrist, I swear she was. I don't think she had ovaries or a uterus, I think she was incognito...
What I can't imagine is what it's like going through what you're going through and at the same time losing people who you love and who make-up your support system. That is a situation that is not healthy and must be horribly painful and lonely...And there is no way that a person who has never had a parrot as a part of their family would ever understand that this parrot was every bit as important to you as any human being is. Some people don't understand the connection to any pets/animals at all, not birds, dogs, cats, reptiles, anything. They think they are "just animals" and don't understand why you're mourning their loss. Again, these people most likely never had a pet, at least not one they were close to, and a lot of them who did have pets were taught by their parents that they were "just animals" to be kept outside, treated like garbage, abused, neglected, etc. So again, it's an inhumanity that is passed-on from generation to generation, unfortunately. Again, you just have to make the choice to either ignore what they say, or if you're more of a vocal person, like myself, lol, you'll tell them just why your "pets" are just as important to you as any humans are, and ask them to please respect that, even if they can't understand it.
As far as Kizzy goes, yes, it's healthy to mourn for her, as right now she is gone. But it is true that parrots are reunited with their families after flying away weeks, months, even years after they are lost. And I'm not saying this to you to give you false hope or to stop your mourning period, but simply because it's a fact. It happens all the time. There are people all over the country that care and that are willing to go out of their way to help others find their lost pets...Just yesterday I was helping a family in the Salinas, CA area learn how to look for their lost 9-week-old baby cockatiel that they lost on May 12th...and I'm in State College, Pennsylvania, how many thousands of miles away....meanwhile, there was also a total asshole who lives within 5 miles of them who found a cockatiel in his backyard on May 9th, and not only didn't bother to look for the owners, but actually posted a video on YouTube about the bird, how he had re-named it, and how it was rightfully his because he found it. The irony of that is not lost on me, nor is the patheticness and the sadness of it. But the point is that there are people willing to help, there are probably hundreds if not thousands of people who were made aware of you losing Kizzy, and if just one of those people see her somewhere they are going to do something about it for you and for Kizzy. People do care. Not all people are like your doctors. Hell, if I saw a BFA flying around in central PA I'd immediately check to make sure it wasn't Kizzy, even though the chance of her being thousands of miles away is slim to none, I would still check. Just like I'd do the same if I saw a GreenWinged Macaw that I thought could be Maggie from central Texas. At this point it could be Maggie in Pennsylvania, and I for one will never forget that Maggie is still missing. So while you are in mourning for Kizzy, you also need to remember that all is not lost...not yet.
And while I understand WHY you feel like you can not get another parrot, maybe not ever, and I really do understand this...Well, let's just say that as a person who has gone through a horrible, serious illness and all of the issues that came along with it, financial, interpersonal, family issues, etc., my pets are probably the #1 thing that have gotten me through it...Now my illness was short-term, but really any issue I go through in life is helped by my pets. So what I'm trying to say is that someone in your situation, who is having to deal with a life-long illness that can be more debilitating mentally and spiritually than it is physically at times, having a parrot by your side to help you get through it and get through each day is not only giving you a support system, but it's actually healing, both physically and psychologically. So I wouldn't write-off bringing another parrot into your life, into your family, not just yet. Not only could another parrot bring your life joy and healing strength, but you too could bring the same to the life of the parrot...