New to This

I have never done anything like this before but don't know what else to do. I am hoping that some advice will be helpful. I am only 24 years old but have been with my boyfriend (who is the same age) for over 5 years. We both are currently in graduate school, live together, and have been seriously discussing engagement. During the past 2 years, he has changed for the worse. More specifically, he is moody, takes things out on me, flirts with any and every girl he gets the chance to in his school (and constantly denies it saying that he has a "friendly" personality), and constantly makes me feel bad about myself. When we first started dating, I was 100% sure that we were meant for each other. However, now I find myself constantly thinking about how my life would be with someone else. He usually treats me great and my family loves him. He is generous and can still make me laugh but it seems as if the bad is outweighing the good. His main concern is himself and getting attention from others. Bottom line is that I am not happy in the relationship. I keep telling myself that things will get better but they haven't for the past 2 years. I am just very torn.