VERY good question...one that we all struggle with.
While I don't know the answer, it certainly helps me to realize that no R is perfect (romantic or otherwise). There will be no one in our lives that we become close to that does not hurt us, or does not have the capacity--painful memories with most all close Rs (parents, etc) Sooooo....here's what I do...
With H and my M....each time I get into a spiral thinking of how I was wronged and hurt (which, of course, only leads to other similar thoughts, leading you to believe that there was NOTHING good...and goodness, how in the world did I marry him in the first place??? ). My tactic: I try to steer my mind to something painful I did/said to him that made him feel the way that I did (hurt, insecure, etc). It helps me to realize that I am not without faults, that we BOTH hurt each other. While this doesn't necessarily make it RIGHT or even better, it at least forces you to remember that you have no choice but to forgive, because it's what you're asking for yourself, isn't it?
Silly, but helps me remember that we're both human...we have such a great capacity to bring each other the most intense pain and the most joy...why not shoot for the latter?