How to stop feeling jealous over a person I don't date anymore?

I broke up with my bf a week ago or so, it's harsh, but I try really hard to feel.a little bit better. But the one thing which immediately ruins my mood is jealousy. When I start thinking that my ex might be talking to a girl he has been chatting with when being cold towards me, my heart plummets. Just the thought that he might like her back frustrates me a lot. It is really hard to let this emotion go and I have not found any valuable information on how to cope with this yet. So, are there any ways to stop feeling jealous? It's one of the few things that keeping me from moving on