I would be very uncomfortable with the person I'm dating flirting with someone else in front of me. Are you guys dating? Or just like each other? I'm sorry you're experiencing such emotional pain over it. I'm not sure I have any advice. Maybe do something healthy that makes you feel better, or distract yourself. If you're dating, I think it's appropriate to tell him that that kind of behavior from him makes you feel badly and you'd like him to stop. Hugs.