How can I be less of a pushover?

I'm just TOO compassionate. I'm nice to people that really don't deserve it. If someone asks me to grab something from across the room that they are too lazy to get, I'll do it without even thinking. I have comebacks and it's not like people can bully me, I can get vicious when I'm mad. but I'm rarely mad ever, and I'm just such a pushover most of the time and I'm too nice. I'm so nice that those people who aren't very liked by others cling to me and being around people I dont like makes me miserable but I dont have the heart to say no to anyone. I CAN say no, but i just dont have the heart to stand up to someone and tell them to leave me alone because they are annoying. I wish I had that kind of confidence because there are a lot of annoying people who do cling to me and I hate it. People take advantage of my kindness and I rarely think of myself and my feelings. I only think about others, only just a little too much. I feel bad for people too much and do things that dont necessarily work out for ME in the end and gets me in awkward situations. How do I fix this?!