I have SO been there!!!
I agree with the things most have said. It'll fall out because of anesthesia, the trauma of surgery, the malnutrition, stress, etc. All of those things have probably happened to you to some degree because of your situation. I lost most of my hair as well. My ponytail was the thickness of a pencil.
I would say don't get extensions. If you're sewing them into the little hair you have, more will probably fall out.
However, wigs have been a lifesaver for me! I used to work at Disney where I wore wigs for almost everything I did (I was a dancer), so I knew a lot about how to put them on and style them and stuff. I made it into a fun thing and tried to be positive. I found a lot of different ones and they made things better. about a year after surgery enough hair had grown back that I was able to stop with the wigs. If you have any questions for me I'd be happy to answer them. I know how honestly heartbreaking this is. After all we've been through you'd think this wouldn't be such a big thing but it is.
Just know that it WILL grow back. You just need a plan for the interim.