Please help my gf and ex wife hate each other and I'm afraid my 8yr old son will suffer?

i'm engaged to a wonderful women but i'm afraid she maybe pushing herself into the stepmom role to soon.she means well but this drives my ex crazy.for example my gf wants to go to parent/teacher conferences(which i love but my ex hates) with me.my ex is a very bitter vengeful controlling person who made my life hell when i was with her.my gf is totally opposite and i would love her to be involved with my son.the problem is this makes my ex crazy and and i want to do whats best for my son.i have told my gf to back off on the stepmom stuff for now but this has made her upset.she feels i should stand up for her to my ex but i just see it as making it easier for my son by going along with my ex for now.hopefully when she finds a bf things will get better but in the mean time how should i handle this?