jealous of my ex

So me and my boyfriend broke up a few months ago, it was on mutual terms and actually didn't have a bad ending. We just decided what we had wasn't healthy for either of us, and we were going to move on our separate ways. He is now talking to someone new. I still don't want to be with him, but for some reason I'm so jealous of her and I don't know why. I know this girl from when we used to go to high school together and I have absolutely nothing towards her I just can't get over that he is talking to someone that's not me. I won't do anything to interfere with their relationship, I'm not trying to be vicious. I just can't control how I feel about it. Has anyone else felt this way or am I just crazy??? 😞
We were together for 2 years, and I just can't seem to move past it