I have been where you are and it isn't pleasant. Nobody is perfect 100% of the time so there must have been aspects of his personality that weren't so great? If so, start by making a list of those things. He left you before for a month, rather than work on it so that is one point to get down on paper. And now, he has done it again without a single warning. He knew how upset you were the last time, so he should have made damn sure that he wanted to make this work with you before coming back, yet he's done it again.
You're probably having a lot of thoughts at the moment about never meeting anyone else like him, never being interested in anyone else or feeling like you did about him, please know that although this seems very really, they are just irrational thoughts, and you will get through this.
Just take it one day at a time, or a hour an a time if that's all you can manage. You have your son, and he is the most important person to you. Can you get out and do things with him?
It seems hopeless right now and the only thing that is really going to help is time. Time seems to pass so slowly when you're in this state of mind, but in a few months you will be feeling a lot better and look back and wonder where the time went.