Being the nice guy in your relationship is a mistake

27 years old here and freshly off of my second long term failure. First one was four years and he left for the guy he married. This one right at seven and he met someone that made him realize all of the things we were missing. In both relationships I have been above and beyond...loyal, loving, committed, always put their needs/desires first. I've been supporting the guy I'm with now (the one that doesn't feel the same) for the entirety of our relationship. Everything he's ever asked for he's had. I just don't know what to do. It seems like people really don't like the nice guys. How do you turn that off though...like...I'm not good at being a prick, but apparently need to figure out how. Geez