My daughter is 34 and I still feel responsible for her as far as being there when/if she needs me. She has two little ones, so that certainly factors into everything as well. There's a question to ask yourself.....who says your responsibilities are "done" when your kids turn 18 ? It may well be that there are grandchildren in your future, and like it or not, they will need you too.
Hopefully you're just going through a "poor me" pity party and your selfishness is just momentary.