I takes time to mourn the loss of a relationship, You just have to go through the process, there not really a shortcut to take.

I would advise you to write in a journal about all your feelings,, it often helps to get it off your chest , like hopefully your posting here is helpful.

As for staying friends.. It's possible, I am friends with a few Ex's. but that was after the break up and a few months if not more of no contact... I see no way to go from a relationship to a friendship from the get go.

I hope you find some peace