My child father allowing new girlfriend to disrespect them

Me and my child’s father have been on and off for the last eight years our oldest son is five are youngest son is three he spent about three years in jail and he is currently living in a group home situation where he will be completely done with his treatment plan next month and he has always been a cheater and that was really the bigger part Of why our relationship is so where it’s at right now but I have been really upset about it and really sad that he has chose not to come around in the last couple weeks at all like he used to see your kids every day spend time with those be a decent family man and I got sick I was septic in the hospital and not one day that he’s been making sure I was OK prior to me going to the hospital or after But in the course of me getting sick I found out that he had a new woman in his life that he was with why I was fighting for my life so a couple weeks has passed and he still doesn’t see your sons arm my youngest son is acting out really bad he’s taking his anger and frustration out on me so I’ve been trying to explain that to their father sometimes I get a text back sometimes I don’t I had a birthday dinner to go to and he did come and get them and everything was fine when I got the back but this time he asked for them to come with him for the day to get their hair braided which I really didn’t want them to do but I let them go for the sake of my kids missing their dad and you know him asking I don’t wanna keep my kid away from their father I asked him for pictures and I can tell in the pictures at my sons were truly unhappy and then when they came home the first thing that that they said was mommy daddy‘s new girlfriend said that our daddy doesn’t love us and they were fighting and arguing I’ve never met this girl my kids never met this girl and I’m very frustrated and irritated because my stomach has been struggling very bad about their dad being out of the home and I reach out to him and I tell him what’s happening and he pretty much tells me that they’re lying that didn’t happen and he is defending this girl over his own kids my sons are unfortunately too little mouse vomit and they tell absolutely everything so I don’t think for one moment my sons will lie about something that would be that upsetting to them but now I’m at the point of where do we go no what do I do I’m very upset as it can’t be asleep about it I feel like I let my son down as a mom so I just really need some advice on where I should go from here and how I should handle this and how would you handle this it’s taking everything I have not to strangle this girl but I have to do what’s best for my kids but I don’t wanna be a vindictive baby and I don’t want To hurt my kids but I also don’t want to allow something like this to go on with my kids and something happens to them and I also know that she doesn’t have any kids my baby father is 23 going on 24 and this girl is only 19