painful as it sounds, sometimes we never get closure/answers it's a sad part of life we need to learn to somehow accept and move past. magickally you could try divinations like skrying, pendulum, runes, tarot, or oracle to gleam some understanding.
i have had my fair share of pain [abusive childhood, parents had a nasty divorce, badly bullied by children and teachers, and my first relationship was was simply missing physical abuse to win the 'worst relationship of all time' award] many of these are events i can't exactly resolve with the person, so you could cram the pain deep down inside and pour booze over it until the pain goes away [that's a joke fyi] or you could use it to grow.
there's a number or TinyBuddha articles i read in the past that really helped me [i'll link some bellow] meditating, journaling, writing a letter to the person/situation and burning it all really helped. [especially the last one, you just have to be open to it. i remember rolling my eyes and writing the first letter but then something in me snapped and i couldn't write fast enough. i was crying and cursing, and by the end i felt like a weight was lifted] while i'm not a fan of 'self-help' books, i do find Thich Nhat Hanh be very insightful, you might find something to help with one of his books. you might also consider designing your own ceremony/ritual as a way to say goodbye and close the book on this particular chapter of your life so moving forward you can heal and not dwell as much on the past. shadow work can also help if these are buried issues over a long period of time.
I found some closure in burning letters. Pour your heart out in your words, seal the envelope, with the equinox coming up, that would be a great time to rid yourself of all your grievances. Either burn in a bonfire, if you still have burning restrictions like we do in Canada, just use a match and a garbage can. Say a few ritual works before lighting, "I rid myself of this person, feeling, etc for the good of all" and set it on fire. Good riddance to bad rubbish.