I can really relate to your feelings of burn-out. That happens when we give & give & give, and we forget to take care of ourselves.
As Geoff said, none of us have to say Yes to something we don't want to do. I do understand how hard it is though because you really want to say yes, but you just have nothing left to give right now.
There is a quote I've heard: "'No' is a complete sentence". But for a lot of us, even though it is just one tiny little word containing two tiny little letters, it is the hardest word to say.
I have been a people-pleaser all my life, so i totally understand how hard it is to say no, and also how exhausting it is to always be saying yes!
It's only in the last few years that I've started to learn that it's ok to say no, it's ok to have needs of my own because they are just as important as other people's needs & it's totally ok to protect my own boundaries & not have everyone walk all over them.
It's difficult to start with, but the more you practice, the easier it becomes, & you can reclaim yourself and your energy.
People don't like it at first, but it's not because you're doing something wrong by saying No. They just don't like it because they are so accustomed to you saying Yes to everything they want, & so they expect you to now *always* do as they want, and it's a shock to them when you now say No.
Saying No & setting boundaries does not have to be offensive. You could try saying things like, "I'm sorry, but that's not going to suit me today" or "Unfortunately i don't have time to do that for you this afternoon", or "i would have liked to help you out with that, but i have other things planned tomorrow", or "i do enjoy babysitting your kids, but i have other commitments on the weekend". If they persist, you can practice firmly but politely restating your "No".
As you are burnt out right now, that in itself should come into consideration. You can say things like "I'm not feeling 100% just the moment and i need to take care of myself, so i won't be able to do that for you this time, I'm sorry".
After all, you can't give to others when you have nothing left because you've already given it all away.
What do you think about some of those ideas?