how long is too long to reminisce?

I (20F) have been in a relationship with my boyfriend for a year now in about two weeks, however i still find myself thinking about my ex i left a year and a half ago. sometimes it’s often, other times it could be a once month, but no matter how much time goes pass he doesn’t go away. when my relationship with him first ended i didn’t take it the best, i think it’s important to mention he was my first real relationship ( meeting families, sleeping over, going out consistently), but i thought about him everyday. now a year into a new relationship, i still find myself thinking about what use to be. it starts with remembering our start, the warmth of the beginning and ends with me being angry with him again for how everything ended. i know this is wrong, i shouldn’t allow the past to have such a hold on me but i don’t know why my mind goes back there. i wish i could move on.