I used to be like that. I remember the first time I said, 'No' to someone asking for a favour they looked absolutely shocked and deeply offended. They took it personally as I always said yes to everything anyone asked, however knackered I was, however over scheduled.
There's a couple of things you can do to quickly turn your situation around.
IMO, the easiest one is to start honest-whinging. This is a trick where as soon as someone asks you to do something, you say. 'You know I can't right now. I am so worn down. I've been over-committing to all sorts of things and running around after everyone else and I just desperately need a break. You understand.'
No one can argue with that one. It is true.
If you have the balls, learn to simply say. 'No, sorry. That doesn't work for me.' If challenged, keep your cool and say, 'You'll have to find someone else. I can't do it.' Long term, this is a far more assertive and empowering method than honest whinging.
To harden your heart, take a long look at how often people go out of their way to support you when you need help. If they don't, you need to feel no guilt at ditching your help to them.
To improve your self care, ask for help. Some people ask for help all the time and others give it all the time. Rebalance this. Think about twhat you need help and support with and ask for it - whether it's professional mentoring, medical advice, a mate to discuss an issue with etc... just learn to ask.