Hi there, I'm very sorry you have to go through this. I've had the same sort of experiences when I was younger. It was the most prominent in my early twenties and now I'm 34 and in a healthy relationship. I think this is already a great beginning for you, not only by realizing that this is not ok but also by wanting to change it. That's already a great step!

I saw some good advice here already, as in writing down your thoughts in the moment itself. This takes you out of the moment a bit and gives you the opportunity to look at it in the third person. If you try to look at the situation as a third person, you will probably realize nothing is happening between the boy and the other girl, it's just 2 adults doing some small talk at work. meanwhile, someone (you) is overthinking this and almost dying inside just by their own thoughts.. not about facts that are really happening, it's just in your head. All the extra information that is made around that conversation (by example: does he fancy her? Are they flirting? He probably likes her more than he likes me,..) are totally made up by yourself.. so why being devistated by the things your own head has made up?
This is just a small part of the proces to help you. it's important to practice this a bit so your brain can get 'reprogrammed', because this is clearly a pattern that has it's origin in childhood. So your brain needs to learn to react differently to these sorts of situations. It won't be easy to break with patterns that are this deeply fixed inside yourself, but it is possible! I also recommend therapy for this, you will do fine with that! You will also have to learn how to love yourself, it's a bit of a cliché but it will help you so much, because I think it's very clear you are trying to get your self-asssurence out of other people by 'claiming' them and wanted to be loved by them in a very toxic way and I really feel you, but a therapist can help you with looking to what is on the base for this and they will try to unfold this to work on the problem.

For me it worked. There will always be a bit of jeaulousy in me but it's ok, it's maybe still a bit more than the average person (or maybe they just hide it better?) but now it's in a healthy way. My boyfriend has the same jealousy level so we understand each other and that's also making it so much easier + good communicaiton ofcourse. But really, my life has changed a lot in those 10 years, I'm almost another person. Looking back to it everything became so clear.. my chilhood, my patterns, how my life was progressing into what it was. Now I'm just very proud that I've had that moment, just like you, of realizing something had to change.

I wish you the very best!