I think it's essential to understand that after a breakup, particularly a less than friendly breakup as most are, you may not get the answers you seek. By that I mean your ex may not permit you to ask questions or to provide answers to your questions. That is very common.
"I have nothing to say to you and I don't want you to call me any more."
That may be the only response you get.
So what then?
Your only recourse may be to examine everything you know about your ex and about the relationship, and to generate various hypotheses ("guesses" if you like) as to what happened, what went wrong, why the relationship ended, especially if your ex was the one who ended it. Include yourself and your behavior or actions as well as those of your ex in these scenarios where it seems relevant.
Then examine each of those hypotheses in the light of what you know about your ex. Which one seems the most plausible? Which one seems to explain the most about how then relationship evolved and how it ended?
That may be your best answer, or at least the best answer you are going to get. Use that to move toward some closure on the relationship and to regain some control over your life again, instead of being buffeted by all the questions and uncertainties you have.