I am a big believer that if you say yes, don’t say yes and then resent it and cast yourself as a martyre. Just say no. If you do say yes, then do it with an open loving heart without resentfulness and judgement. There may come a time in your life when you will need the help and your own “angel.”
I agree in principle, but I think OP's situation is less that she agreed to something and now resents it, and more that she is constantly being asked to do more than originally agreed to and the mom is not offering anything in return except things like weekends away that OP does not want and can not "use."
Saying no can be so hard, but sometimes it just has to be done. I have a good friend - a really close friend- who asked to arrange regular alternating sleepovers for our sons because she and her husband like to have regular all night "dates." (I get it.) But this is not something we need (we have a very young child as well so we do not really get anything out of the overnights except exhausted cranky kids.) But we did try it, and it just does not work for us for a variety of minor reasons. It was hard to tell this very good friend that I had to stop this arrangement, but I did. And our friendship has survived just fine.