Don`t regret it, regardless.
If you feel resentful, let yourself feel it. If you feel bitter,..feel it.
Think to yourself as many nasty name-calling things as you can, vent to someone else,..whatever you need to do.
..but once some time passes, let yourself take the lessons. All relationships teach us something about ourselves. Especially in poly, where (thankfully) books cant totally brainwash us,...the relationships themselves teach us.
So you had a relationship that didn`t work out. A first poly relationship.
You now have the realization, that you might not of known exactly what you wanted or needed going into that, but in the aftermath, I bet you are a lot closer to knowing what you want and need.
You have learned, and you now have experience.
You are a armed person, in that way.