Yes, ask yourself why you’re holding on to this jealousy.

Specifically, imagine there’s some part of you, some corner of your psyche, that thinks it’s really important to stay jealous and fixated on this other girl and find out once and for all if he’s really really really over her. Then ask this part of your psyche (you can imagine it as a creature or animal if you like) Why? What would happen if I wasn’t jealous anymore? What would happen if I let go? What does it get me to hold on? What’s the benefit of holding on?

And please, you’re not being crazy. You’re just stuck on something, or something is stuck in your head. You recognise it’s not rational and it’s bothering you. That’s normal enough. And you are here asking for advice on how to address it or work with it or learn to let it go. That’s totally rational and you’re taking care of yourself. So try giving yourself a little credit when you start to feel like “Ugh why can’t I let this go already!!!???” You’re working on it. You’ve worked at getting stuck on this for four years. It may need a little time before it’s ready to loosen. Be patient with yourself.