It's been a year and a half since I split with ex DP and I feel stuck, as if I can't move on.
We were together 4 years and it was great for the first 3. We talked in terms of the future very early on, he told me that he saw himself marrying me and having children. We would go on weekends away together, he was fun, helpful and everything I'd ever wanted in a partner.
Then things changed. He stopped making the effort to go on days/nights out with me, neglected his appearance, was dismissive and distant. I thought that maybe he was depressed. He didn't like his job, felt like he was going nowhere and so I helped him rewrite his CV. I'm sure that mentally, he left me long before he actually did.
About 3 months before we split up, he went to his work colleague's wedding reception, alone - I was originally meant to be going but he said he had made plans to give a lift to other work friends so there would be no room in the car. 
Conveniently, his phone lost signal at the wedding reception and he didn't come home until 4AM. Photos of him in a nightclub surfaced the next day on social media (with his workmates, all in smart wedding reception wear) and he explained it by saying they'd all gone out clubbing after the reception ended. I was a bit suspicious but let it go.
When we split up, he said out of the blue that he felt things were not working out with us, he wasn't willing to discuss it as he felt it couldn't be fixed and that was it. I was shocked. He moved out of our flat the next day.
I later found out he'd been seeing a married woman behind my back. She left her husband for him and they were together a month after we split up. Within 2 months, she was pregnant and they now live together with their baby. I have no doubt that he was at her house the night he went to the nightclub. She lived right next to it.
I can't forget all that's happened. I can't believe he's moved on so quickly, without a thought for my feelings. I still feel betrayed, even though it's been over a year and I've tried building things into my life: I've made new friends, got a new job, tried new hobbies but I still feel empty without him. I miss the things we used to do together. I miss the trips we had and the little jokes we had together. I miss feeling like I always had someone to rely on if I needed him. The feelings get worse at this time of year as his birthday/Christmas were close together and we'd always do something special for it.
When I think about him with her, I get a dark, sad feeling that lingers for a while. I feel so sad about it all. I've tried dating other people but I don't get the excited feeling I used to have when I liked someone before. I'm always worried that if I trust another person, the same thing will happen again. Sometimes I imagine what we could have had but then I remember he's chosen to leave and have that future with a stranger he'd probably only just met. I feel resentment towards her too. She was married to her husband for 5 years, had her own life and home yet she (and he) chose to ruin her marriage and my relationship.
I probably sound insane, still thinking about this person after a year and a half but I am genuinely worried as I've tried to move on but it's not happening. He's long gone, yet I'm still stuck thinking about all the things that happened and wondering why he did it. Thanks for reading.