Why does this keep happening in my relationships?

No need for intimate details, but, basically, I’ve had two serious relationships and one that has the potential. With the first one, everything went great for a year and a half, and he was much more committed and in love than I was. I would get angry over stupid little things, and he would try everything to make me happy. It didn’t work and I left him because I felt like I was missing out on the freedom of being single. Another guy came along, and despite all the reasons why we shouldn’t have been together, we were. Again, about a year in, I kept getting mad over stupid things that he couldn’t control. I started fights for no reason and just stopped feeling anything towards him other than contempt, again thinking that I was missing out. Now onto the third one, and I see myself doing the same thing. I don’t want it to happen, but I’m worried that it will and I will hurt him, too.
I know that the problem lies with me, not with the guys that I choose. They have all been amazing and would have done anything to make it work. I wonder if I’ll ever be content. Anyone have similar situations? I know I should just be happy, and some days I am, but some days I just want to start over again. Am I just bored and not yet realizing that relationships aren’t all unicorns and rainbows?