You sitch sounds like mine from way back in the day. I had a GF and although we weren't engaged, we often talked marriage. Then one day she dropped me like a hot potato and went off with another guy. I was devastated. I did everything wrong, and for awhile I was her friend, but each time she left me a crumb my heart got torn a little bit more until I finally gave up. I joined the Air Force and met my future wife.
Three days before my wedding day I got a call from my old GF and she told me she had sent me a letter and it was important I read it. I got the letter the next day and in it she was begging me back and told me she had been missing me like crazy for the last few months. She couldn't get me out of her mind and said she was stupid for leaving me. I seriously thought about breaking it off with my soon-to-be W and going back with the GF, but I knew I couldn't do that and eventually she would break my heart again and I could not imagine the thought of hurting my W. I read the letter a couple more times and threw it away. I didn't contact the old GF.
I never looked back and to this day I don't know what happened to her. I never told my W about the letter until a month after my BD (I wrote her a 19 page letter then).
It's strange, but even though it happened 32 years ago, it does give me some hope for my current sitch. I completely detached from my GF then and later she came coming back to me.
It might be a pipe dream, but it's a dream I'm still holding out hope for.
M 55 W 52 MR 32 T 34+ D29 BD May 8, 2016 - She moved out ILYBNILWY May 15 (Through email) No EA/PA August 23 - DB used against me in every way Divorce July 18, 2017 - Life is getting better every day