I can answer the HIDA scan part, as I'm having one next Wednesday. Basically, it's to see how your gallbladder copes. I have to have a fatty breakfast, and then I'll be injected with a type of dye to help them see how things are working. Then I have to drink some kind of drink that is apparently the equivalent of a really fatty meal, and they'll watch as that goes through. It's basically to pinpoint where things are going wrong.
What you need to do is understand that none of that is going to last forever. The operation will help your hand. The tests will help the doctors figure out what else is going wrong. Do you take anything for your depression and anxiety? If not, get on top of that straight away.
I was in a similar place after falling off my old mare a couple of years ago. I was in agony in my elbow, I couldn't straighten it and it was always sensitive, not to mention the pain if I bumped it on something. 3 months after the fall, 3 x-rays and a CT scan revealed that I'd fractured my elbow, effectively snapping off the tip of a bone inside my elbow. I had to be ridiculously careful with everything to keep myself pain free. When they finally operated last year, it turned out that there were actually two pieces of bone, the biggest almost a cubic centimetre. They'd been there long enough to cause some damage, tearing up the tissue and cartilage in my elbow. That same fall has also left me with permanent, degenerating back problems, so I know where you're coming from.
Put your studies off until you've got your health under control. Continuing now is only going to make your depression and anxiety worse. You're only young, you have plenty of time.