How do you stay friends when you break up with someone and you have a child together?

I currently broke things off with my boyfriend of seven years and made him move out of my house. He had been planning a wedding with someone else while he was living with me. This is the second time he has planned on getting married to this girl and I found out both times from the engagement section in the newspaper. The guy is a jerk and liar. I know I have to be civil to the guy to show my son a good example. But how do I stay his friend after all he has put me through? And how do I get over this so I don't ruin my next relationship with my trust issues?? totally confused and messed up here!How do you stay friends when you break up with someone and you have a child together?
Okay first off.....just try not to talk bad or down on him or blame him for things in front of your child. Second your probably never going to forgive him for what he did to you, but just let sleeping dogs lie. If you keep dwelling on it your never going to move on with your life. Start hanging out with friends more or go out. It might sound cheesy but get a hobby...anything to get your mind off of him. It's just going to take time..Honestly he probably didn't deserve you and he is probably going to ruin alot of his relationships down the road. Just move on and the right man will come your way when you least expect it. Just go out and have fun now!