I agree that it’s all misguided and if you think the DC did not pick up on this resentment, you are probably mistaken.
We all have tried to help you see things from another angle
There is another angle, you just refuse to look at it.
It is clear you did do all the hard graft when they were small, and he wasn’t very present.
Whether they were mistakes or bad decisions- He appears to have tried to rectify this in later life by adapting his life to incorporate more of the DC and spending more time with them, building a relationship with them.
During the early period he left them with you and helped raise them by providing financial support to you.
I know it’s hard to move past how resentful you feel that he had this freedom, but you did have all those wonderful formative years with your DC and money can’t buy that. You benefited from what he lost, although it doesn’t feel like that now as you have lost the children you gave up your freedom for.
This man may have been selfish in the past, no one is disagreeing with it. What we are saying is that your plan to make him pay for these years of crap support was also selfish and self centred. In the eyes of the DC, One of you (him) has moved forward and learnt from old mistakes and tried to be a better parent to the DC, and one (you) has taken a downward spiral into bitterness and it’s clouded your judgement terribly and harmed your relationship with your children.
You need to talk to a therapist about your bitterness and resentment towards your ex so you can process it, as it’s literally eating away at your life like a toxic cancer. He isn’t eating away at your life, he is just living his life - you can CHOOSE what you let affect you and get to you. You can choose what you do now. You can choose to let this go or hold onto it