How to not shut out friends but not go insane either?

So here's the deal. I'm pretty sane about my current state of things, which is not splendid. I've been clinically depressed before, this is bad. BUT I'm kind of, sort of dealing with it (though wanting to kill myself most of the days, but no stress, I've got two elderly parents to take care of, I can't afford to kick the bucket) and let's just hope that getting out of it, maybe even this year. The irony, though, is that my closest friends are having the time of their lives – one just bought a house, another has gone on a dating spree with really great men (she deserves it after ending abusive relationship), third one finally got pregnant after 4 years of miscarriages. I'm madly happy for them, they are few of the closest people I have. The thing is, I find myself physically unable to go meet them and be all bubbly around them, since I can't be bubbly around anybody if don't leave myself at home (I can't :( ). I'm also not the type of person to ask for help, so I'm dealing with my stuff alone and they only know the fact that it's happening.

So how do I keep being a friend and also not make myself worse if any social situation is pretty much like running a marathon? I don't want to shut them out of my life but I'd rather be by myself for a while. Sorry for my Tarzan english, it's late.