rosamundjohn: you need to break the cycle here, Octavius is right-as often :)
It isn't wrong or bad to want time to yourself, in fact you deserve it, and people who are heaping demands on you should value this. If they don't, they aren't your friends, and you don't need them anyway.
I know it is difficult, but start by perhaps making a list of all the things you do that you don't want to do any more, and maybe some alternatives that are centred around YOU.
You can't please everyone-it's almost a fact! So by trying to do this, you are placing yourself under so much pressure, and still not achieving your goal. I am not putting down what you do; I think it is an admirable quality to want to be the person who does thins for others. Rather, I am trying to show you that you need to give yourself a break.
No matter if it is personal or work related, you have to speak up and tell people you already do enough, and can't fit in any more, sorry. Have you thought about an assertiveness class? It would show you that it isn't rude or inconsiderate to say no, it just ensures that you don't take too much on.
So next time someone asks, "would you mind just...", take a deep breath, look them in the eye and say NO! You don't even need an excuse, you shouldn't have to justify your own feelings, and if they ask, just say you have a lot on at the moment, and can't add to this.