Is the resentment that builds. Over time, this is the quicket way to end any relationship.
And there is no clear antidote to this.
You can obviously try couples counseling, reading books, talking about it, think of a systematic approach to minimizing conflict…there are plenty of paths to take.
However, I do think at some point you have to find a way to forgive. You lost that time/opportunity with it. And you need to come to peace with this.
The best thing to do is to go forward. Resentment doesn’t just eat the past, it also consumes the present. Creating a negative spiral.