There are a lot of times they don’t want to go to their father’s even for every other weekend visits
How much of that is due to you though? I have a friend who refused visits with her father once she reached high school. After having her own kids, and reflecting on the situation, she realised that the choice she made was wholly due to the mother’s behaviour at the time and simply wanting to make her mother ‘better’. That realisation was 20 years on and then caused a strain in their relationship, and a rekindling of the relationship with her father (which had been non-existent for the 20 years).
I also think you are downplaying your part in this ‘tragedy’. It seems you were with a man who refused to marry you, as this is what you wanted but he didn’t (and no, that’s not the same as relationships where neither side want to get married). That says he is not wholly into you, not completely committed to you and is basically there for convenience only in case something more suited to him comes along, as this is typically a man’s thought process. Yet, you chose to have children, and more than one, with this man. That’s on you. Did he enthusiastically want children, or was that something else driven by yourself and he pretty much acted as a passenger with someone else driving the bus? If so, that would have been another flag that was ignored.
Now he has found someone he is into, wants to be committed to and actively wants children with. It wasn’t you, and that must hurt, but realistically that’s not his fault, and you likely ignored the flags when you had the opportunity to walk away up front rather than driving the bus off the cliff. Your children shouldn’t be punished by your hurt in this regard.