Sue -- I know what you mean about you "not wanting them to leave you." That's more of my fear when my good friends ask me to do something for them, while if a stranger asks me, I'm more afraid they will hate me if I say no.
black_mamba -- Your example about the physics homework will help me in the future
usually if somebody asks me to do their homework, I do it without thinking about it - but they can't get mad at me if I just help them, can they? And if they don't get what they want, they'll just stop asking.
I think one of the problems with the fear of having people mad at you/upsetting people by saying no is that it's not all that realistic. Sometimes, I look around and I see people saying "no" all the time. One of my problems with that though, is they seem to be "good" at saying it, they're casual and nice about it. I think that me saying no would be so unnatural that people would be like "...whoa wtf" instead of responding like they would to another person who they are used to being said no to. (if this makes sense...)
AnthonyJ -- You seem to have some of the same problem I do. "In private, I get really pissed off at myself and at the world, but in public, I have a hard time with asserting myself and standing up" I know exactly what you mean by that, it can be very frustrating... I also feel like a time bomb, except maybe my timer's messed up, cuz whenever I think I'm going to go off, I back away from it and say "okay... I'll do that"
I saw this book called How To Say No Without Feeling Guilty -- it seemed okay, except it's not really that I feel guilty, it's that I'm worried others will be mad at me.
Thanks for the replies, everyone