After decades of overstretching myself to try to please everyone, also personally not wanting to miss out on fun stuff...
I could/would never say no without feeling the need to justify and explain in detail. Younger, being part of a big team, also friends, I wanted to do everything but did find myself going to events when all I wanted to do was not get ready for another night out. I had the energy though, even after DC.
Then there suddenly came a point when I suddenly wasn't as interested anymore. This had to do with a few different things; grief most of all, DC with severe autism became clear, micro management and pressure at work, problems to almost divorce with DH. Perspective of life changed, I changed and my priority became me and coping and doing the best for my family.
I stopped organising trips for my school, didn't attend as many events, minimised my after extra school voluntary classes. I had been a key part for over 20 years and I thought time for the younger ones to step in. I felt a bit guilty but also relief. I had another job which included evenings and suddenly found a new sense of freedom...
I discovered I didn't need to have to make excuses, I would just say I've got other commitments! School knew about my other job, the responsibility of ND DC, a terminally ill parent.......my other job (call centre) knew the same. Suddenly it became so easy to just say I can't, well because I couldn't.
It's a shame it got to this stage of burning myself out before realising this! Now, I still have both jobs but when asked if I will be attending X, even though I may be free, I simply reply no, knowing they understand I have other commitments. It's not a lie, I may not be at my other job, DC is in school, the other commitment is ME and time out.
Of course if it's mandatory I go but anything else, past that stage!