I helped my GF through the turbulent end of a friendship 2 years back. Turns out it was the end of an affair she was having. How do I recover from this? [28M 26F, together 6 years]

Me and Haley started dating in Uni. It was always a great relationship with very little incompatibility, and the incompatibilities that were there were dealt with well. In 2015, Haley got an opportunity to move to Helsinki for 10 months that was great for her career, since we were both from London it was a massive decision for our relationship. Both of us thought of the other as the person we were gonna marry so there were a lot of tears and heartfelt discussions. Career vs Love. Finally we decided that we'll be a long distance couple, came up with bunch of ground rules about skyping, visiting, intimacy etc.
We managed pretty well. I thought.
She had a great time there and her career went from peak to peak. She made a really good friend 2 months in, Adeline. They used to hang out all the time and she helped Haley adjust to the City and showed her aaround, they went on weekend trips, she spent time with Adelines family, celebrated their birthdays together etc.
Then 6 months later Adeline started dating someone and cut Haley out of her life, which made Haley miserable and so she became really dependent on me for emotional support. I tried to do my best and sometimes would screw up and Haley would rake me over the coals for that, I learnt how to walk on eggshells during our conversations. And I felt really bad for failing her, cause after all Haley just got deeply hurt by a friend in a new country where she didn't have anyone.
Well she came back to London in late 2016 and we moved in together and our lives got enmeshed and we were a perfectly content couple. She did miss Adeline and would talk about her here and there, but it seemed like she had moved on. In late february me and Haley finally decided that we were gonna start trying to have a baby. So there has been a lot of talk about best ways to get pregnant etc. and she has been posting about that on her twitter.
Last week a random guy called Luka followed me on twitter and then quickly started DMing me. He Dmed a bunch of pictures of him and Haley, screenshots of their conversations on a variety of apps. They were graphic and sexual and romantic and literally unbelievable. I didn't know what to think, so I merely replied WTF? He told me that he and Haley had been dating for 6 months while they were in Helsinki together, he was also a part of the same training programme and Haley had led him to believe that she was single. they had supposedly been dating since the first week of the programme and they broke up because Luka met someone else.
I was decimated and was in complete denial, so I went back to mine and Haley's chatlogs during that period and all the sundry details matched up. The trip to a spa resort she took with Adeline was at the same time as the trip Luka took to that same resort with Haley, except this time there were pictures of Luka and Haley topless in a hot spring. Those pictures that Haley sent me of her birthday party, well Luka was actually in them, but there were a couple missing, specifically the ones of her kissing Luka.
I mean, ****. Those pictures of her trip to Adelines family home, that was ****ing Lukas family home that was Lukas mother and grandad and sister. I never wondered why there was such a lack of Adeline in those pictures of Adelines family. But now I know.
But here's the worst part. I HELPED MY GIRLFRIEND THROUGH HER BREAKUP WITH HER BOYFRIEND SHE WAS HAVING AN AFFAIR WITH. AND i took **** for not helping her "correctly" for not being there at her beck and call when she felt really down. I made the mistake of having my own career and job responsibilities and not prioritising my cheating girlfriends emotions through the end of her affair.
I printed a bunch of them out and left it in our flat. I took some stuff and moved to my friends place. I have blocked Haley from my phone, but I called her from my friends and told her that we need to meet in a public place on Friday evening. she didn't come. Instead I was graced by the presence of three of her friends bearing a letter from Haley and advicing me to go to our flat and talk this out and come to reasonable settlement of our 'tiff'.
Seriously.
They called it a tiff.
the letter was basically Haley blathering on about a big mistake and she was lonely and homesick and depressed blah blah blah. She wants to set it right and will quit her job "to be your slave, even sexually".
I never realised the amount of hatred I could hold in my heart. We are over and I'm gonna make sure that she understands that we are over. I've gathered a group of friends around me who are cooler and calmer and we'll go over to the flat and take my stuff and leave. So that is not an issue.
THe issue is how do I respect myself again, I was duped that easily, I trusted her enough to never even check for Adeline on her facebook, she's there, except Adeline was her superior and their pictures were from professional events. I allowed myself to be screamed and nagged at because I didn't respond to her tearful snapchats about her heartbreak. I Can't fathom how I let myself become such a doormat.
I don't know how I can ever have any self-esteem ever again, how do I ever get into a relationship without becoming a possessive, paranoid prick.
TL;DR: My long term girlfriend wnt abroad for nearly a year for her work. there she made a great friend, fell out with said friend and turned to me for emotional support. Which I gave to the best of my abilities but not enough. Turns out the friend was a brand new boyfriend she decided to get herself while lying to me. It's been two ears, we decide to have a baby. That boyfriend shows up in my life and destroys it. How do I not spiral into non-existence?


Source.