The marriage has been on the rocks for a while. I think that is more or less the way you describe why you separated in december. He has met someone new, that is nice for him. She is not someone who has run out on her husband to be with your ex, she lost her husband. She confided in you that she had several miscarriages, that is a pretty hard thing to do. Her message, maybe poorly presented and clearly poorly received that was that she will treat your children as if they are her own as the relationship moves forward.
I don't know if knowing my history will help at all. My wife dumped me for another man a year ago, she had been having an affair. We have two children 7 and 11, we share residency 50/50. I met someone 11 months ago, started dating 8 months ago and moved in together (my two plus her three) just before Christmas (after I got my
Decree Absolute). I let my ex-wife know before we told the children (my new partner did the same for her ex-husband, separated for three years now) out of courtesy.
There really was nothing that either ex-partner could do about our decision, but my wife actually became very nasty. Mutual friends that met my new partner told my ex-wife how nice new partner was etc. They thought they were helping, but it actually made things worse. I think the fact that my new partner is very capable, has three children (similiar ages to my own) and teaches at a school that one of my children (and one of her chidlren) goes to was actually very threatening for my ex-wife. My ex-wife will always be my boys' mom. And, I feel she should be happy that when they are not with her they are with me and someone that treats them as her own.
Likewise, my new partners ex-husband has watched two other men come and go, never felt threatened, but has really been rather nasty about me. The children are not allowed to mention my name when they stay with their dad. Both ex's seem to go out of their way to find fault with us. So, the ex's did not want us, but they also don't seem to want us to build a new family. I guess they feel very threatened.
But why the lies?
Well, how about financial. Maybe he did not want you to know she had a place in spain on the basis that if might cause you to think could get more from him. Trust pretty much breaks down in the early stages of a marital breakup.
Frankly, his choice of partners is really none of your business, ven with children in the equation. Nor will his concerns about who you might have as a partner in the future carry any weight.
It is very easy to be critical of a new partner on the basis taht it is concern for the children, but really it is more often facing up to the fact taht we ahve been replaced by our ex-partner and they itnend to mvoe forwrd with tht person into a new family unit.
My advise, and I am dealing with thread and your other one, is let the kids go on holiday at half term with their dad. his new relationship may or may not work out, but if you trusted him to take care of the children when you were together, you have nothing to fear now.