How come i always feel guilty afyer standing up for myself?! ?

People always tell me that I'm really sweet and that I'm such a good person and a good friend. I try my best to be all of these things, but there are times when I feel like people are taking advantage of my kindness and take it for weakness. I can stand up for myself, but when I do I always feel guilty like I'm doing something wrong. After I've said what I needed to say...I always question myself...like was I wrong...should I have just not said anything, etc. Why do I have this guilty conscience? I think I'm afraid that people won't love me anymore if I express my unhappiness with their behavior. I know I need to set boundaries for people, so they don't mistreat me, but why do I always end up feeling this way afterward? Alone, outcast, sad, guilty....like a bad person? I really need someone's advice..