phrase requests - Is there a word for "What might have been"?

Context: You made the decision not to see a person any more. The relationship was good, you were both happy with one another but the one main stumbling block (for you) in the relationship was never dealt with. It remained, a permanent fixture, that forced you to make that sad, and final decision.

Is there a word, an idiom, or a famous line that expresses the sadness for something that might have been; e.g. a rich and fulfilling relationship. It's not nostalgia, because that expresses a wistfulness for the past.

Instead, you are wistful for the future.

Answer

Perhaps acceptance captures some of the sense you're seeking (see painful acceptance below).

You try, and you try, and you try, until you reach the point where you can try no more. You have slowly come to the certain realization that you will never get what you want and need, that there's nowhere to go with the relationship, that there's no future.

You have two options: continue down the road to nowhere, or call it quits. You're not going to want to call it quits unless you believe, with all your heart, that you've done everything you could to make it work: that's the only way you're going to be able to sleep at night, knowing that you did everything you could.

When at last you reach the point of calling it quits, and you make the painful decision to end it, you resign yourself to the outcome, to a new and different future. You don't want to do it, you really don't, but you have no choice. The die is cast.

That's acceptance, at least as I imagine it in the context of your question.

You could call your feeling of "what might have been" a painful acceptance. It looks both backward (painful) and forward (acceptance).