There are two really crucial ideas that are relatively new in the therapy field, that anyone in a relationship needs to know. They make up the the systemic, relational paradigm shift that for all its value, and having been around for decades, could be missed! To miss it would be like missing out the penicillin and micro-organism knowledge if you had an infection when that was just taking hold a ninety years ago. To embrace this relational paradigm is more important than the exact approach one uses, though it needs to be a relational one. Imago, Psychodrama, Non Violent Communication and many other approaches are systemic and relational, or at least not actively opposed.
One is that the right here, now, in the relationship is the solution to the relationship problem. How to get there might be painful and hard, you will need to learn skills, make effort, but individual therapy or leaving, or searching for a better mate has all those problems and will lead to similar relationship problems, or to no relationship at all.
The second is that it takes only one person in the relationship to commit to really working on it. In fact the ability or desire to take that role is never even and equal, so it is never quite fair.
These ideas seem straight forward to me now, but they fly in the face of much more prevalent notions, ones I was actively taught, and took on board as wisdom, and have had to unlearn: That it is good to sort yourself out before you have a relationship, and that each person needs to commit to doing their share, that it is 50/50.
Al Turtle puts all these things very well. Great to find his site today.