Don't worry, this is perfectly normal.
Our minds go into set patterns after a major emotional trauma like this.
Your mind wants to set everything in order, with explanations, in order to assimilate the experience, learn from it, and move on.
As you are discovering, this is an erratic process, which doesn't start, proceed, and end neatly.
You may find that there are parts of this situation that never feel 'resolved' in your mind, but with time, your mind will stop trying to figure them out, and simply accept them. Time is the key here, but unfortunately, there is no set pattern or length to which you can refer - we are all different.
What you need to do is go with the flow.
On the occasions when your mind puts this situation to one side, and leaves it alone, then go with that. on the occasions - which vary in strength and frequency - when it takes everything out and goes through it bit by painful bit, you need to go along with it, but be willing to put it aside again as soon as you feel able.
You will find that as time passes, the intensity and frequency of the feeling of needing to know what happened, and why, will diminish, and the rest of your life will love into those spaces, which allows you to move on with your present, and not endlessly fret about your past.
It takes time, but be patient, and be nice to yourself. This was not 'your fault' so don't beat yourself up with reasons, you may not find any.
Put your energies into your new relationship - your ex didn' think your relationship was worth hanging onto - you should make sure you feel the same way.