I am 'the other woman' in my situation.
My DH had a long term relationship that he split from amicably not long before meeting me. They never lived together, got engaged, or married, despite the long relationship.
Within 2 years of meeting me, we had bought a house together, married and I was pregnant.
I remember asking him why he had never taken any of those steps with his ex. He said that while he loved her and respected her as a person, that he wasn't 'in love' with her. They had just plodded along together and because nothing was fundamentally 'wrong' as such, they stayed together almost out of habit. She had raised moving the relationship along and moving in together and mentioned marriage and it was then that he realised that he didn't want any of those things with her and that she wasn't the right person for him. There was nothing wrong with her. She was, on paper, perfect for him, but that he just didn't feel that she was the person he wanted to spend the rest of his life with. He didn't see marriage and children in his future with her. They amicably split and are still friends now.
He met me and we clicked and have been happily married for 25 years.
To be honest I never stopped to think about how his ex felt when we moved in together and married so quickly. But I can imagine she maybe had similar feelings to you at the time.
I think another PP put it well when they said that you were just not meant to go the distance. You were not each others 'person' and that is fine.
It is just a shame for you that he didn't figure this out earlier and allow you to be free to meet someone else.
There is still time for you though, don't lose hope in that.