Is it a bad idea to contact your ex to find out why they treated you poorly?

Please don’t give this person the satisfaction that they got to you, and yet you still came back to talk it through. The absolute best thing to do is to completely cut off all contact, move on with 100% commitment, and then they’ll be the one wondering why they behaved like such a shitbag.
That way you’ll give yourself a really good chance to be happy and fulfilled, and they won’t matter anymore.