Unchien - the limbo [censored]. You can't move forward and you can't move back. If I was a different kind of person I would have followed up on the opportunities that the universe has put in front of me. But I am not. I have not yet let go of hope and until I do, any spark of a new relationship will be extinguished at the first hurdle. I wish I could let go. There is not even any resentment any more. Just a lingering thought refuses to go away hat we are meant to be. I wish I could blame him for keeping me in this place, as many have suggested I do. But I can't. More fool me they say.
I suspect the resentment you speak of will pass and be replaced with understanding and then eventually a kind of ambivalence. The person you loved and then hated and then loved again with such intensity will become just another person you once knew. You will remember certain bits fondly while other memories will have memories that still sting. But there will be a numbness to it. You will hear in passing that they have got a new partner, moved in, got married, and each one will sting a little. But then your mind will turn to something else. The weather, the trains, the report you had to do, that good looking new colleague that has just joined the office, and the feeling will pass.
So, in answer to your question on how to let go of resentment, my response would be (and appreciate this is harder then it sounds) to a place of understanding. When she does something that causes the resentment to rise try and think of what her motivations might be. When you come up with a negative motivation, replace it with a positive one. If you can't do that, then take a long breath, try and hold your head up, and wait for a calmer moment to call them up on their behaviour.
For H mentioned to me in the car (with D13 and D10 in the back) that he was planning to take the kids away to his mums holiday place for three weeks (he would be there for ten days and the children would stay on their own with his mum the rest of the time). I remembered instantly how I felt when he did this same trip last summer. He had done a number of family holidays without me (and each hurt) but those three weeks really hurt. So I said no. He could take them for two weeks, but three weeks was too long. I felt his body stiffen but he didn't say anything. At the same time D13 said she wanted to stay at "grandmas for the whole of summer". I felt like the wicked witch of the west. Now, 12 months ago, he would have huffed and puffed, given me a dirty look, and then started saying I was selfish. But all he said was "I was thinking about childcare". I thought about this last night and realised that yes, he was thinking about childcare, he also probably thought about how much the children would enjoy it, and, going to his mum's is friendly on the purse strings. He was not thinking about taking my children away from me. He wasn't thinking about me at all. So, in the middle of a text exchange today, I told him I understood his reasons for wanting to take the children away, I am not trying to deny the children time with his mum BUT three weeks is too long, AND could he not discuss holidays in front of the children until he has checked with me as it makes me look like the bad guy when I say no. He said "OK. I understand. D13 is difficult and that three weeks is too long". No harm. No foul. He respects me a little more for standing up for myself. And I don't hold any resentment for being made to look like a [censored] in front of the kids.
I read something recently that kind of sums it up. "Do no harm. Take no [censored]". It's a balance. But it's possible (I think).
I am of an age when many of my friends are separating/divorcing. I cannot understand the behaviour of their partners, or at times them. Petty stupid stuff. Turning their children's phones off when they know the other will call. Cancelling arrangements at the last minute, posting stuff on social media that their children could possibly see, fighting over every last dime on their financial settlements. Your behaviour during this time will stay with your children for the rest of their lives, it will inform how they deal with conflict and will influence every relationship they have. Your comment re moving beyond being good co-parents is predicated on her wanting to be more. And you can't control that. Be grateful for what you have - being good parents right now is enough, and it is more than what many others have.