Bit of back story.
After 14 years and 4 DC we separated 9 months ago.
Nobody else involved we just stopped being a couple and both were miserable.
We have remained good friends, co parent the DC, parent the same way and are 50-50 regarding DC.
EXDH has been dating for a few months ( I helped him with his dating profile 
) but nothing serious and I have been in a relationship just under 6 months. This was fine as neither of us feel that way about each other however ex has met somebody he has really clicked with and now there is a problem.
Two weeks ago I received a text from ex saying he had introduced DC to his new friend But he thinks the DS 11 may have twigged there was more too it so may ask me questions. Now this came as a shock as he had only been out on his first date/meeting on the Tuesday , second date Friday and introduced her to DC on the Saturday. My instant thought was this is too quick but I trust him, he is level headed and has always put DC first. I text back ok no problem I will manage any questions from DSS.
I collected DC a few days later and DS said he had met dads new GF I asked if that was ok and he said it was fine. DD's 8 also said the new she was dads NG as they were holding hands and dad touched her bum 
.
Long chat with DC about how adults need relationships and that is ok as they will always matter to us and relationships make people happy. They seemed fine with this and all said NG was nice and chatted to them lots.
Fast forward 7 days and I drop DC back at EXDH quick chat, relationship going well had a few dates/stayed over all very positive. I collected DC 4 days later and there was something wrong. They were cagey/quiet when I asked about their time with daddy, did they have fun etc. I didn't push them to talk but later that evening DS11 and DS13 asked to chat. They said they are upset about dads NG. Their first day back with dad NG came over in the afternoon and stayed all night. They said she is nice but it felt weird (first women in family home since me) her sleeping in my bed and her dressing gown in the bathroom ( I have no idea why this bothers DS11 but it did). She stayed the whole day Sunday and then came for tea on Tuesday. So out of the 4 days with dad they only had him On his own for 1 day.
I told them it is natural to feel weird about it and that I would have a chat with dad. Inside I was fuming, and couldn't believe he would rush this with DC. I spoke to the DDs the next day and they echoed the weird feeling and they like her a lot but wish she wasn't always there.
I rang EXDH and although I tried to stay calm I am ashamed to say I lost it with him. He had not once thought about DC and was only thinking with his dick 
. In between ranting I did manage to point out the affect his actions had on DC. He refused to believe he had done anything wrong and said I should trust him. I told him I couldn't as he had got this so wrong and his priorities were clearly not the DC.
He called back a few hours later apologised and said I was right, he and NG agreed they rushed it and will back off with DC and let it progress at DCs pace.
He said he will talk to DC and apologise/ make it right with them.
As I said at the beginning he is normally level headed and a fantastic loving father and I know he just got caught up in the moment and made a mistake but I can't seem to stop being angry with him. Throughout the spilt we worked hard to make it good for DC, they see us together being friends laughing getting on and this has had a positive affect on them. Right now I don't even want to look at EXDH but I know that this would cause problems for DC.
I need to get over this and move on but it feels like the anger is here to stay and I don't know how to get rid of it. Anyone able to help me calm down and move on from this?
Sorry for the epic post.