Dude,
You knew eachother for less then 2 years, but I know how it feels to have your FUKIN GUT RIPPED AND HANDED to ya.
My ex and I dated for 31/2 years, she had a 16 month old when we started dating that I fell head over heels for. I paid her rent, bought her a car, took care of Anne when she needed time alone and was completely,deeply,madly, head over heels for her and her daughter. The night before I was to propose to her she said she was going to the movies with a girlfriend and I went out with my buddy. Gil and I were walking down the street headed to a pool hall and as I look across the street I see my boss coming out of a restaurant with her, she is all over him.........my heart fukin sank like a rock. It took 3 years to get over her and during that time she would still let me see her daughter(I loved that child)
We have become friends but I will never forget the feelings I felt for those three years, it was a pain I thought would never go away. Writing this brings it back a little but whatever you do, know that it will go away and your life will continue. Don't harbor a resentment or hatred it only prolongs the pain.
Good luck my friend