I've done a lot of personal work on taking care of myself, saying 'No" when I feel it is in my best interest, taking time for me, getting honest with myself when I am feeling resentful, etc.

I can tell you that when I began this project it was not comfortable at first to say 'No' or 'Let me think about it and get back with you'. It does get easier with time but initially it is scary.

I did learn that there are gray areas to operate in. Sometimes a flat out 'No' is what feels right. Sometimes I can negotiate ... 'I can't do X but I can do part of X or I can do Y instead' or 'I can do X if you/others will pitch in and help', etc.

(Too bad the requirements for the conference volunteer position were not spelled out more clearly - if asked to do what seems unreasonable over the required 15 hours, then you can try some new boundary skills.)

Kudos to you for recognizing your issues and wanting to try a healthier way.