I know this is the first time I have posted,however I just wanted to let you know the deep amount of respect I have for you for doing "the next right thing." I think the gym is a great place to vent all the negative shit your going to be going through. Once you get over it,your gonna look and feel great!
I know what your going through. I met the love of my life some years back and decided in no uncertain terms I was gonna spend the rest of my life with her. Then came the night before Christmas eve. I caught her with my own brother! Those two ended up getting married. When I was informed of this I responded with "Congratulations...you two deserve each other!" That was 15 years ago and they are divorced (both cheated on each other) and miserable.
Your approach is much better than mine was.(I got wasted for the next 3 years and burnt my life to the ground!). Keep going to the gym. Do what you need to do to deal with all that shit your feeling. In the end you will be MUCH better off!