Then about a year after committing the wheels start to fall off. You then feel guilty about what you did to your ex, and think maybe he wasn't to bad. That guilt makes you reluctant to "pull the trigger" again, so instead you stay and slug it out feeling you made a mistake and bear a load of resentment on your shoulders. Your partner senses this and the whole thing spirals.
That is a good picture of a bad situation. Here is something better to strive for:
Resolve to behave as well as you can in your current situation. Enjoy the good parts of it. Do what you can to not make things worse, and give him room to improve if he is ready to. If you live up to your values and ideals, and are honest with yourself about what you want and need, you will be able to either make a relationship work well, or see that it isn't working and move on.
A healthy middle road between giving up and running away, or sticking around and just getting hurt.