Left a narcissistic wife and fell straight for another

I spent nineteen years married to a woman I knew was borderline, but seemed to develop ever more narcissistic traits as time passed. We have two teenage boys together, and for years I debated leaving the marriage but always decided to stay for the sake of the children, and I guess because I was afraid of being alone.
Eventually it became clear that I had to leave. My wife's behaviours were dramatically impacting my children's lives. My youngest is very introverted and my eldest started doing drugs and stealing cars. The crunch came when my eldest said to me that I can only stand being with their mother because I spend time away from home at work.
So I left, and having been isolated from my own friends during the marriage confided in a female work colleague I'd been helping through a work disciplinary issue. At first I presumed we'd never be more than friends. There was an age gap, she was 29 and I 43, plus I saw absolutely no signs of attraction.
As a couple of months passed, we started seeing more and more of each other, and I started to develop strong feelings for her. After a while I just had to tell her how I felt and was amazed she felt the same way. From the word go this was a full on relationship. We virtually lived together, were inseparable and when apart there was constant texting and skyping. I'd never felt so in love, and after an abusive marriage, I'd never felt so valued and appreciated for who I am.
My new girlfriend can't have children (she has PCOS) and the children were suffering terribly post separation as their mother poured all her anger onto them. My new partner encouraged my to take custody of my children, but I was initially reluctant to do this as it would mean moving them from their home in France to London to be with me/us.
Naturally I saw it was in the kids' best interests to do this, but as I work away from home a lot I would be in need of support, and we hadn't been together all that long. The warning flags should have been seen when she placed ultimatums on me with hindsight....... if she hadn't met my children by a certain time she'd be gone. There were constant double-binds where she'd say she never thought she'd ever meet a man as wonderful as me, but then immediately follow that in the next breath with "if you don't show me the respect I deserve I'll be gone, no explanation, I'll just go". Also she's cut all contact with her parents for reasons she never really explained, and not just them but all friends and family members who have any link to her parents, so they can know nothing of her at all.
In June she met my children and they got on like a house on fire. She took my children in hand and really helped them through their problems. I'll always be grateful to her for that. Then they both decided they wanted to come and live with me/us so they moved over, we looked for and rented a flat and moved in. August and September were wonderful. I could see she wanted to create the family she felt she didn't have and couldn't make of her own, but that was fine with me as we were all so happy.
Then the devaluation started. She started becoming paranoid me and the children were ganging up on her. She became agitated at any contact between the children and their mother, and seemed to want them to cut contact with her too. She became critical and sometimes angry with me, as the relationship wasn't as focused on her as it was before, she became more and more controlling and possessive of me and took to some inappropriate displays off affection in front of the children.
Still, I thought it's just a transitional phase and we'll be fine in time. Then one afternoon she tried to phone when I was out with my eldest. I couldn't answer as I was in a meeting, but that was it. A sarcastic text and when I got home she'd packed and gone. All she said a couple of days later was "you haven't shown me the respect I deserved so now I have none for you". I have no idea what she meant by that, and guess I never will. I sent some emails after the split to try and figure out what happened, but she's simply ignored me. My eldest sent her a happy Christmas text and got brief but cold reply.
It's been three months now and I'm still reeling from this like it happened yesterday. How can someone who was so in love, and who I loved so much just discard us so suddenly and inexplicably. Most of all I can't believe I left one NP had read all about it, but fell straight for another. I let my kids get hurt again too. Finding this forum has helped a lot, and I know I'm not alone.