Running into him and seeing him all the time certainly doesn't help. He has to be completely removed from your life for you to really move forward, which is clearly not happening in your scenario. I know you love him and care about him, but as long as he cannot face the consequences of his actions (having you or other girls no matter what he does) he will never change, he will never learn, and he will never grow up. Not that it really matters to you right now because even if he does change, your history would always be gnawing at the back of your head of the things he's done.
All you need to do is focus on you and do what's best for you. Having him in any kind of buddy buddy situation is not good for you and your progress. If he tries to come up to you again, you have to establish that you need space from him and he should respect that. Taking a look at your relationship here, you know that it's a bad one and you aren't happy. You cannot help how you feel, but you can control your actions. You have the ability to say no to him and you are more experienced with this kind of behavior and know that it's wrong. Logically, you know you deserve better and there are so many other people out there that are willing to treat you well and better than him. He is a bad choice and he will face his own consequences eventually, don't you worry about that. It makes you feel better to see him down as sick as it sounds, and he will eventually hit that bottom.
Anyways, it's tough being on your own when you had somebody for six months and now you are left with a void. Don't be afraid to be on your own for a while, and find your happiness without somebody. You shouldn't NEED somebody in your life to be happy, you should want them to be. I don't think you are ready yet. It may feel like a competition and he is winning being with other girls, but he is not growing up and his bullshit will stink the place up the more he pulls it. Spend more time with your friends, do things you enjoy doing, concentrate on your school work, maybe be more active to get yourself in better shape. You will always still think about him but the more busy you are, the faster time will pass by. And time is the thing that heals you here. The more happy you are with yourself, the happier you will be with somebody else in your life to enhance that happiness. Your self worth is not based on his egocentric behavior. You have to feel worthy because of your good qualities and what you are doing with your life. And it will exude from your attitude and behavior and it will attract somebody else. All in due time.
Waking up next to a beautiful girl,
Step outside and say hello to my beautiful world.