Can you fully accept your SO as s/he is without judgement, if you know him/her to have this personality trait? No? Then let her/him go before you vex yourselves.
If you want to be able to accept it or if you realize you’re overreacting or making out friendliness for your partner trying to disrespect you and looking to seduce someone right in front of you (which is not usually what’s going on, OK?), then you have to drill down into the core belief that makes his/her behavior bother you.
Is it that you don’t feel worthy of a partner? Is it that you feel ugly? Is it that you feel your SO flirting is a reflection of your worth? Well, it isn’t. So every time you see that your SO’s flirting triggers the thoughts, “I am not worthy. If I were, s/he wouldn’t be flirting,” STOP IT! Intercept it. You must pay attention to what you’re thinking and block the feedback loop.
This is not easy. But you must do it. Changing a core negative belief is like quitting smoking. Every minute you must focus (in the beginning) to not slip and run out for a cig when the stress button is pushed. After a while it gets easier.
Your bad habit was reinforced over time. That’s why you still have it. Reinforce a new, more positive belief. “I am worthy of a decent partner’s time.” “My partner cares for me.” “My partner is being friendly.” And openly communicate. Let them know it bothers you and that you’re working on it. Trust yourself. If your partner really is one of those people who needs to be constantly swarmed with sexual attention from a lot of people, then you know to go. If not, then help yourself to stop hurting yourself with awful beliefs.