Funny thing - once there was a time where my friend was litterally knocking on my dorm room door, while my light was on (I was most likely inside) AND I had JUST TALKED TO HIM AND TOLD HIM I was in my room like 1 minute before. Ya.... and I never really explained why to him either. So ya. Been there.
Sometimes, though, even though you don't want them friends are the best thing for you that's the nature of the beast - It makes you want EXACTLY the opposite of what you need to get better.
Maybe, talk to one or two that you trust, if you can, and remember that you don't have to be "happy" like them. People often seem happier than they actually are, and I know you're hurting right now and want to be better but you can't compare your happiness to other people's happiness.

I know you don't want to tell them what's going on, maybe a few of them would understand if you said something vague like, "I haven't been feeling well" or, "I'm sorry I really can't talk, I'm not feeling well/I"m tired??" That way you're not ignoring them completely?

Loads of hugs