You are not being selfish at all, but you need to be taking better care of yourself. Your mother’s needs will continue to eclipse your own if you don’t. No one else can do it for you. Your mother’s labile mood will deplete you if you don’t make a plan to nurture yourself first. Allow yourself a chance to say “no” regularly and let your mom know what you can and can’t do. Be aware that her disappointment doesn’t mean you have to alter your life from top to bottom to make her feel better. Let her have the dignity of struggling to manage her own disappointment. It isn’t your job to make everything better for her.
When you say no you will most likely feel guilty, but the resentment from being depleted that you will feel if you keep giving into your mother’s emotional needs will be so great that eventually you’ll have nothing left for her.
I am not suggesting this will be easy, but learning to take care of your needs first, while limiting how much you give to your mom is essential for both you and her.