I feel the same way as you do really. I'm finding it easier, but that's only because I don't even bother to share things anymore.
It's really hard to be the supportive one when you're falling apart on your own. And it really isn't fair when friendships only go one way.
Unfortunately for both of us, it's probably because we let it happen... I don't know about you, but I just keep on being the support for people who are never, ever, there for me. Even when I really don't want to, I just seem unable to do what your friend did and say that I am not available at the time.
We probably both have "Use me!" tattoos stuck on our foreheads.