How to deal with excessive jealousy?

I have trouble with feelings of jealousy. Not just in romantic relationships, but for crushes and sometime friends as well. It is not something that leads to unhealthy external reactions (trying to control the person, etc), but on the inside, I have trouble calming my feelings and this causes stress on my daily life.

One exemple: there is a girl at my job. We have been working together for around 6 months and, during most of the this time, our relationship was very troubled. We would ignore and sometimes be rude with each other. At one one point, we were not even saying hello and good by.

This changed 3 weeks ago, when we started to get closer and now relations are cordial: we talk a lot and go smoke together. However, there are some other coworker with whom she is closer, and she is always flirting with them, specially if there are people around.

I know that I am been irrational and it is a very stupid feeling to have, but I feel a very strong feeling of jealousy every time she do this. I feel like a idiot for it.

How to deal with excessive jealousy?
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