It's no fun to feel like you're the sole source of emotional support for your mother--that can be very frustrating!
The first step is to figure out what you are and what you're not comfortable with, and build some limits and boundaries from there, so when you are on the receiving end of an emotional bomb, you can respond with empathy (which is what your mother is looking for), but stay firm.
Have you had a chance to check out SET (
TOOLS: S.E.T. - Support, Empathy and Truth) yet? It's a good way to communicate what is OK for you, but also recognizes the other person's point of view.