How do I move on and enjoy my current relationship as much as i should

I dated this one girl, and we broke up on semi decent terms, but I never officially lost feelings for her. Now Im in another relationship and its goikg really well, but everytime i see my ex doing anything we used to do with another guy, and with clear signs that she's moved on, it makes me jealous and holds me back from enjoying my current relationship as much as I usually would.