Ex keeps canceling custody time last minute and Im losing it

This is the third time in 6 weeks that she's canceled my weekend with the kids because of some work thing or new boyfriend plans. Meanwhile Im blocking out my whole schedule, sometimes rearranging plans with my girlfriend, and then she just texts me Thursday night that they cant come. The kids are confused and disappointed and honestly I think she's breaking the custody agreement. Do I need to lawyer up again or should I try talking to her first? This is making me want to just go for full custody at this point.