I used to feel like this Katek but when you do reach a stage when I truly can't do something then I do now say no to people, even an appointment at the hospital last week. It's no use feeling guilty, it gets me nowhere and I really can't help the way I am. Same with taking my son tomorrow, I can't do it, I took his sister but I was able to then, but now I can't.
You need to take reasonable care or else your body will start to tell you that you can't do much at all. I have pushed myself over the years and tried to be all to everyone and it has got me nowhere. My kids are awful to me, my mother would have had me lie down and her walk all over me (and it still wouldn't be enough) and my siblings looked down their noses at me no matter how hard I tried to be a good sister. Even friends have taken more than they have given in the past. So now it's a no and I look after me. With the exception of GN meets, those are on the calendar.
When my son goes tomorrow, my phone will go off for a while while I get my head sorted and have a few hours to myself.
Learn to say "no" into the mirror, it takes practice. And you don't need to say why neither sometimes, just no, sorry I can't do that or I won't be able to get there etc. Try it with small things first and build up. But do take care of yourself, you only have one you. X