stbx started seeing someone within weeks of my moving out

My stbx told me she wasnt interested in a relationship(wives are leased mules) then within two weeks of me moving out she was dating. the whole time we were together she complained of my smoking & drinking beer. She is currently dating a guy old enough to be her father he smokes & drinks. She has been bringing my children around him which hurts me tremendously & i dont think it is very good for kids so soon into this & have told her that its not good for them to be involved in her dating but she continues to do whatever she wants. She has phys. custody for now.  i see them more than she does. She didnt take kids on one vacation this summer but has gone on two by herself with her bf.  i dont understand how she went from being good mom to terrible mom so fast. She is 38 & i think it is a little too young for midlife crisis but she ditches kids every chance she gets then sneaks off so i dont find out she leaving them home alone because when i do know i try to spend time with them so there not alone & tell her how messed up it is to do that.  She is choosing this guy over her kids does anyone know if this will stop soon & she will even out so she is home with kids who need her? She wanted divorce I did not.