StarCat's advice is what I do. I cry until my body shakes. For as long as it takes. Then it seems that the sadness and pain have less power over me. Make time to be sad and feel what it is you are feeling. Write it all down if you need to. Write letters (but do not send!!) to him about all the things you are feeling.
The understanding comes in time. It is hard to understand when the emotions are still so raw. Then our rational brain kicks in and starts to put the pieces back together for you. The more you let those emotions out, the easier it will be to get to that place of understanding and acceptance.
Just remember you are worth so so much more than how he treated you. MUCH more.