You might never get the explanation. Closure is something you can sign for yourself.

Some people leave without telling you why, and waiting for the reason can cost you more than the ending did. Hoping for an explanation is really hoping it will make sense, and sometimes no version would. Holding the door open for an answer keeps you living inside the last scene. You are allowed to decide the scene is over without their signature on it.

Write out what you would say if they were sitting there, then do not send it. Name what you are taking from it, even if that is only what you want next time. Ending it yourself does not mean you stopped caring.