Honestly - I've felt this way all my life. I'm usually the happy one who puts on a good front, and most have no idea what's going in my head or in my life. As for people depending on me, that's not really been a problem. Besides being everyone's advocate, I haven't had to be the "strong" one, and I'm sorry that I don't know what that's like.
I think the best thing for you to do is be honest with your friends and family. What you're feeling isn't a joke - it's real. You feel overwhelmed, and feel taken for granted. There is a way to put this without alienating anyone.
Tell those who you listen to what's going on with you. You don't have to be specific or divulge anything you don't want to - but turn the tables a bit. "Yeah, I can relate to that because this is what happened to me..." or "I understand because..."
Involve yourself, but don't ever expect the same in return. Some people want things to only be about themselves, and there is nothing you can do about that.
I think you are a good friend, and that's probably why so many depend on you. But it can't be about them all the time. You're human, and you have problems too that should be heard.
So I suggest you speak up. No one is a better advocate for you than you! Reach out to them the same way they have to you.
You need to take care of yourself. You can't take care of anyone if you don't care of you.