Losing a pregnancy is very traumatic for a female. It took my well over 5 months to just stop crying at a drop of a dime. It took over a year and a half to come out of her depression. Though, did she find validation in another man? NO. We together, through continued talk and physical signs of support like hugs, holding hands, and just other forms of physical contact showed her that we were in this together. (Non sexual contact).
Your GF made conscious choice after choice since her Christmas party to engage with POSOM. Choosing to validate herself rather then her relationship with you. There were mainly opportunities for her to stop what she was doing and choose you. Especially the first night at his apartment where she said that she only went topless. Hate to break this to you, if POSOM was touching her.... its physical. And she was a willing participant in it. As others have said, adults just dont kiss. And your GF continued to choose her own validation over your relationship together til she got caught!
There is a saying, dating, even if long term, is a tryout for marriage. She FAILED. I know you want to try to see if you two can work this out. That decision ultimately is up you. All I can say being older the you and seeing and living life is... once trust is broken and a betrayal is had, the innocence and trust of all out love is shattered forever. There will always be doubt in the back of your mind that she may betray you again.
Though this situation hurts, be thankful this has happened before you proposed to her or better yet, married her.
You do not owe it to her, to help her through her depression. You do not have to be her white knight. She fired you as her significant other the moment she allowed POSOM into her life and she continually went to his apartment.
Please make your decision on your relationship with your GF in your best interest, not hers at this point. She has shown you where her priorities are. You show her yours.