fiancé's baby momma drama PLS HELP

How do I deal with baby momma drama? She tries everything to ruin us and even lied to me about them taking a shower together and kissing while I was crying on the phone, Even when my own fiancé's family doesn't like me and tries to help her by trying to get them back together. His own mom helped with that lie too and idk what to think or do he took a picture with her and idk what to believe but.. I am very incesure and very mad. Someone help me with this please