I'm a compulsive-pleaser and I'm bad at saying 'no' to anything. If someone asks a help from me, I say yes, even though I know they don't deserve it. If anyone asks me to finish some thing, I say Yes, even though I'm already stretching myself thin.If someone gives me an extra responsibility, I say Yes, though I know I could use some rest.
Actually, I tried saying No to some people at some instances, and I ended up regretting it later. It makes me feel guilty for rejecting some one's preposition. I feel responsible for what they'd feel after I say No.
This is a manufacturing fault I have, and I don't know how to recover from this without making enemies of people around me.

Any suggestions I could use to smoothly say no or reject something while not hurting their feelings?