Hi, I think the best way to get over him is to stay far away from him, if possible. Starts going out and meeting new people, do not rush into another relationship take your time someone will come into your life. When you think of him, start thinking of the valuable time you are wasting. Get involved with some fun activities. Take Care!
The key to forgetting a past love is making yourself too busy to even think about it. Make yourself busy with your job, visit friends, start going out again. This will definitely help you forget him. Always see the positive side of things. You have mentioned that he did not have a good past and he already has a girlfriend, so I believe he was not worth having a relationship with. You can think on the same lines. And I am sure you would find someone very nice and good for yourself. Best of Luck!!
It is unfortunate and sad that you didn't become aware of your feelings until he met someone else, but I wouldn't give up on him. Stay his friend and wait it out, they may not last. Or if you are determined to forget him maybe you should tell him how you feel first. Maybe he would be willing to leave her for you. If not then forget him and move on.
I once TOO thought that keeping busy was the answer to forgetting a past love, etc. However, in all honesty here, I'm not so sure about this anymore. I tried this too(maybe we all have at one time or another) I will say it DOES help to a degree. On the other hand, places one goes or things one does may ALSO be a reminder of a past relationship. There's also the fact that that person may be your first thought all the time for some unforeseen reason so, I can't say that keeping busy is a DEFINITE answer but I think it COULD help.
Your question was asked 3 years go, I hope by now you have forgotten that guy and you have moved on. I would have suggested that you keep busy by doing physical exercise, arranging your things, keeping your clothes and shoes neat and clean. That way, you would have directed your thoughts to productive use.