Dear Katie,
You have been through a lot, but your anger is not at this jerk of a guy, and your sympathy is not for his girlfriend. Both are at and for you. You made some very bad choices for yourself. You had unprotected sex, chose a sexual partner that did not care about you (I’m not sure why you expected someone that you had casual sex with to support you during pregnancy), ended a long term relationship, and most importantly cheated on your boyfriend. None of this “happened” to you. These were the choices that you made. Focus on why you made these decisions, how not to make poor ones again, and take responsibility for them by not making this about the jerk. Set new boundaries for yourself and others and move on. You don’t need to forgive him, just forgive yourself.