My best friend and I haven't had any classes together this year. It sucks but whatever. Since then I feel like our relationship has been strained since and they haven't even noticed. I would invite them to hang out and they would either leave early for their other friend or ask to invite this other friend with us.
Then they only talk to me about two things, being stressed and cosplay. The cosplay I don't mind, it's fun! But they won't stop talking about how stressed they are and come up with the worst problems ever and will tell me immediately if one comes up. If I ever try to relate to a problem or bring up one of mine, they turn me down and begin to talk about their self again. Twice, they've snapped at me because of their stress when all I do is listen and try to help.
I don't know what to do anymore. My stomach hurts and I just don't want to deal with this anymore.

The problem seems to be with your friend. It seems that your friendship revolves around him/her. All they do is talk about themselves meaning that they care about themselves and not you I guess. If you tell them that to their face they will obviously say "that's not true". Furthermore, the stress thing is just too much. I don't know what kind of stress, but they have to deal with it rather than constantly complain about it.

>>2 Even if the problem is with the friend, I'm pretty sure he/she still wants to be friends with them, when you're pretty much telling them to stop being friends.

Maybe you can kind of, not ignore them, but just wait until classes are over for the term/semester before hanging out again? like once the stress of that is over maybe it will be easier to hang out normally again? depending on what it is thats stressing them that is. because i mean it is insentitive of your friend to be focusing on their problems so much when talking to you and snapping at you like that, but sometimes people do react to stress that way and it doesn't mean you can't still enjoy hanging out with them, if you still want to.

>>3 Well if that's the case, I don't think they are friends anyway.

Look here people, friendship consists of two people not one person listening to another. It's a contribution of both parties. Furthermore, what type of friend complains constantly about their stress only and cuts off the other person when speaking. That to me isn't a friend but someone who is self-indulgent. This doesn't mean you cut them off forever, as >>4 says, you can still maybe hang out with them after. Either that or you can even tell them that you don't like what they are doing. If they are a proper friend they should understand that.