Do you regret any of your past relationships?

Have you ever regretted a relationship? I don't, it's just frustrating how much work and effort I put into something that was doomed from the start. How much I sacrificed just to keep that person happy when there was virtually no chance of us being together long term. It sucks when you look back on a relationship and think of the time you've wasted. I like to at least try to learn a lesson from every relationship, regretting stuff from past relationships is that it means there is still part of you that hasn't learned from it.