You sound like a substantial and serious young man who really cared about this woman whole-heartedly, and was not in the relationship for the "handy-sex", even as a top priority. She was much more than that to you. Because of that, a lot of this "bury yourself in meaningless sex" advice is not going to help, as Jason said. You need to find the solid foundation of your life, and dwell there a bit, and let go.
That foundation will hopefully include an open quiet Church somewhere. Just go in and sit. Healing thoughts will come. The Spirit is always there just at our soul's windows --- open a window up. Thoughts will come about what you still have --- family, friends, work, health, time, ... life will begin to look promising after all. Ways will open to you. You'll begin to be thankful for what you have.
Now the tough part, but the fastest way to healing: forgive her. She messed up. She's making bad decisions. You used to really love this person. Forgive her. Hope that she straightens out her life. Hard...I know...it will come if you let it. Forgiveness is paradoxically more about ourselves than the one forgiven. You, I, we cannot heal the other. But we can remove the lead that has been laid upon our souls. Let her go. Walk lightly. Go into the light --- there's a lot of it out there...and a lot of very nice people who really DO want to be part of your life.
God bless you.