I'm getting sick of my depressed friends ranting all the time
What are you doing to help your friend get out of depression?
I have mentally challenged friends too, that's why I'm stressed out and I don't know what to do anymore.
It sounds like your friends need you, and it might be frustrating, but you shouldn't just give up on them. It is important to be there for people in times of need.
I don't have the heart to tell them no. I'll be their bitch forever.
Don't look at it that way. You should see yourself as someone of value. I'm sure they appreciate everything you do for them and see you as an equal. You sound like you need a break, so let them know you will be busy for a couple of days, then you will feel refreshed enough to help them out.