Ex now ruining my relationship with children

[Yankeelane]

So i have been separated from my ex wife for 2yrs now and we have both seen other people during this time. I see my four children every weekend and have them most holidays. She has had nore than one partner around the kids regularly even having him taking them to school. Now i have introduced my children to the person i am with as i am really happy with her. Suddenly my ex has reverted back to a spiteful and vindictive attitude saying i am not to have her anywhere near the kids and saying i wont see the kids. She even had my daughter text me to tell me i was to either pick my new partner or my children. My ex has now stated she is going to tell one of my daughters that i am not her real biological father even though i have brought her up since she was 2months old and soesnt know any different. This is all out of pure spite because i am happy with somone. My children really liked my new partner but after two days back with their mum she stated that they dont like her. She has also indormed me that my elder daughter who has ongoing mental issues for a few years now has recently stated that she is gurting herself because i am with someone else yet my wx has equally been with other people. I am at my wits end my children love being with me and all the time ask if they can stay and not go back to their mums as they arent happy there. Please help i am trying to be amicable and dont get into arguments with my ex but i need help as to what to do. And i am especially not looking forward to friday having been forced into telling my daughter and probably crushing her. Please help.