I don't know what's going on with me...

I don't understand what is happening to me these days. I'm so confused. So around Tuesday I got into this mood, I sat in bed, I cried, I felt pain, I thought about suicide and I tried to push my friend away because I felt like I bring pain and misery and bad luck to anyone I associate myself with. By Thursday, I realized I wasn't as alone as I've always thought. My friend fought for me, wouldn't give up on me, and was there although I kept pushing her away and that made me feel better. I've been staying with her since then and she's been there for me. No one has been there for me at a time like this. People usually bail the moment I say I'm feeling down, not even try and push them away. That made me feel better, knowing I wasn't alone for the first time, that someone cared, someone was there. And I started feeling much better yesterday. Today I'm still here with her, but I woke up this morning and I was feeling tired and I was feeling that pain so strongly again. The thoughts are back and as much as I'm trying not to think about them and to avoid them I can't get it out of my head. I don't know what triggered it. It's like I woke up and it was there, I'm not even alone. I don't know what happened. We went to the library and trying to study and as I'm sitting here my whole body is shaking and as hard as I try I can't even fake being okay anymore. They can see through me. I keep smiling pretending to be okay because they have exams, they have stuff to get done and I don't want to be a distraction I just want to be okay to finish my own work, to be normal, but I can't stop shaking. My whole body is shaking. They keep asking why am I so nervous, I keep saying I'm not. They think that I'm shaking from stress but I don't think I am. I'm not sure why I'm shaking, but then again I haven't been sure of what the hell is going on with my body lately. It's been acting really weird and not normal. Not my usual way, or the normal human way either. It scares me a little. It worries me a little. I don't know how to deal with it all. I don't want to burden my friends. They've already been trying to cheer me up all weekend and been there although they have their own shit to do, but they've been here regardless and right now they need to focus on their work and I can't bother them. I want them to at least believe I'm okay for now, I don't want to burden them with my problems. I don't want to have problems. I want to be okay. I want to stop shaking. I want to not think about suicide. I want to stop being sad for no reason. I just want to be able to finish my work and get my stuff done. Why can't I just be able to do that. 
I don't understand how someone can have the will power to lose 150 lb but not be able to get over something such as depression no matter how much I want to and no matter how much I try. I just don't get it.