Custody schedule is affecting my job, what do I do?

I have the kids every other week and two weeknights but my new job wants me to travel some months. My ex is saying I need to give up the weeknight custody if I'm gonna be unreliable. I don't want to lose that time with them but I also need this job. The custody agreement says I can work a reasonable schedule but she's interpreting that strictly. Do I need a lawyer to modify or is there a way to work this out?