Sometimes it has little to do with love. Reasons people behave differently in another relationship can be down to a number of reasons
- they've grown up - quite literally they are just older. The hobbies and nights out that were so appealing in their 20s, stop being that way in their 30s (had they remained single, they still would have stopped clubbing eventually type of thing). Or people get bored of previous hobbies and passions
- they grew and learnt from the last relationship. When a relationship ends it generally drives people to reflect as part and parcel of the grieving and coming to terms - whether you're the person who was dumped or vice versa - people tend to grow from a significant relationship ending
- a new partner may not be as accommodating. They set boundaries and they mean them, they may have a better sense of self, stronger support system, high internal confidence. It doesn't mean a person loves them more - but it can mean they'll walk a tighter rope because they understand this person will end a relationship far quicker than their previous partner might have done
While none of that necessarily feels like a 'make me better pill' - it can give some pause for thought. It doesn't mean he loves this woman more - he's just older, a little wiser, and maybe she's much less tolerant of putting up with any nonsense.
And in a new relationship yourself OP, might you not be exactly the same? A little older, a little wiser, and much less inclined to put up with nonsense?
The issue with your child is seperate. My only thoughts would be - don't force a parent on a child that doesn't want to be there. The kid knows and it will hurt longer. Your DH can grow old and bare the guilt of this - which again, unless he's a monster - he will do - usually when the child is an older teenager/young adult and realises what a waste of space and how insignificant their father has been in their life and they'll naturally distance themselves. It will sting him when your child grows up and gets married and gives very little thought to their father being included in the wedding, or doesn't hold a ticket for them to be at their graduation, or doesn't think to call them when they hit a major milestone.
So my advice would be - don't block the relationship, don't interfere either but also don't facilitate beyond efficient co-parenting - e.g dont make the effort to follow up on a parents evening if you know the school are sending the same communications to him - that sort of thing. Do reply to a text organising a time for their usual visitation.
Never speak highly of him but also never speak disparaging of him. Stock responses to your child if they ever get upset about it is - I'm so sorry for you DC, you're such a brilliant child and I love you to pieces. You always have me.
Offer the reassurance, offer the love but don't make excuses or show anger. Stay neutral.
Beyond that OP - it's just time to move on. Allow yourself one last bloody good cry, and say to yourself 'that's enough now, it's enough'. Everytime one of those intrusive thoughts come into your head about them say - I don't care. Say it out loud if you can and immediately see it as a sign to move - so say you're watching television and your mind drifts to thinking about them - say out loud - I don't care, then get up and clean the kitchen/put a load of laundry on/ sort out some clothes for charity - whatever it is - physically move yourself from where you were.
Also use the stock responses of 'I don't care' (said neutrally - without any anger behind it) if anyone tries to share info on what he's doing with you and change the topic of conversation immediately. So friend Y: 'hey did you know that X was away in France last week?'. You: I don't care. Tell me about your holiday plans though? Are you planning on going anywhere?
Sometimes we have to break the cycle on feelings because they can get stuck in a loop. Like some dodgy electrical cabling that needs fixing, it's time to do some rewiring. It really can be that simple.