I truly understand what you are going through, I have been there myself. My advice to you is to leave him and find your own way. I now how difficult your situation is, I thought I could never live without my married guy, I spend 7 years loving someone else's husband just to end up alone. They often never leave there wife, you have to be strong and make this decision for you. Understand that he is enjoying the best of both worlds, he is fortunate to have someone who is willing to change there life for him. You are young, and believe me love will come again. I now it's hard to conceive, because it seems that you really do love this man, but it will. Remember to love yourself first, and acknowledge that you deserve to have a man of your own.
Good luck!