A few weeks is no time at all, I doubt he was over you then. He will be eventually though, and there really isn't much you can do about it.
It takes a healthy dose of reality to help quell those thoughts of you guys getting back together. Telling yourself he's the only one for you isn't very logical, because how can he be the only person for you on a planet of 8 billion people?
The relationship wasn't one that you were incredibly happy with, but you didn't want to give him up either out of fear of being alone. He made many mistakes and could do better. He wasn't the perfect boyfriend.
Logically, if you two were back together now, it wouldn't work out anyway. You both would need to become different people to have a chance of working out, but if one or the both of you were the same person as before, it would have the same result: break up. It takes some serious time to really grow and change from an experience and that's if the experience actually changed him. He was already chasing after another girl, which I doubt gave him proper time to analyze the situation and to start putting the necessary changes into every day life before his next relationship. It just wouldn't work.
Until you meet that next person, you will always be looking back at him. But you will meet somebody else and you will realize that you don't need him in your life to be happy. This experience has actually put you in the position of benefit. You have learned so much from your time with him. You have grown, matured, and have a better understanding of who you are, what you want, and what to tolerate/not tolerate in your partner. You are able to love more as you have a better understanding of what love is and how it feels, so you can love with greater passion. Everything will be okay, all you can do is concentrate more on living life without him instead of hoping he'll spring up and swoop you off your feet and say how much he loves you and how sorry he is.
Somebody else shouldn't be the key to your happiness, but it should be something that enhances your life. Finding happiness on your own first is essential to truly appreciate what the other person has to offer. And when you aren't out looking for it, how funny is that it comes and finds you?
Waking up next to a beautiful girl,
Step outside and say hello to my beautiful world.