You need to have a serious talk with your ex-boyfriend and resolve any issues that both of you may have. You also need to be clear on the kind of a relationship you want to have; so that there's a clear borderline between friendship and seeing each other. When either of you gets another partner, work at fostering friendship so that everyone remains happy.
you need to figure out what went wrong? you need to have a deep conversation with him and tell him exactly how you feel and ways to fix the problem..... then in the future if you want to get back together than you need to figure out the best way to do that....you should both try dating other people because then you realize the problems that went on between the two of you. you got to think what did you break up for? what was the problems? if both of you wanna be together than maybe you just need to take a break. get back together and like start over and start at the start