In my opinion and experience, you are grieving the loss of one....while latching onto another. And somewhere in the mix is the feeling that you do not deserve better.
I would suggest getign out, meeting other people, but let yourself work through the grief. Rehashing with him is not the best way. Cutting ties is often better, especially where abuse is concerned. But the best place to start is looking in the mirror. Not that you did anything wrong, but to see why, what is it tht makes you feel so desperate for the boy? Is it because you feel you only deserve to be treated wrong? Maybe seek some help with self-worth. There is a wonderful book that you can get on online, "The Significance Syndrome" it is a small book that is easy to read, but had profound insite into why we do the things we do.
I would talk honestly and openly with the "fiancee" and keep it as just an ingagement, until you process all of the things you need to in order to have a happy relationship. Rebound relationships are often our desperate attempt to just not be alone in the world.
Make a list, divided into 4 columns, what you need in a relationship, what you want in one, what you can negociate adn what you absolutely will not tolerate. Be honest. When it is in black and white, it is easier to see what we are chasing adn why. But stick to your list, if they do not fit it, move on.