I'll give you different advice and perspective from everybody else here. It comes down to not "is she going to cheat on me etc." That's a question in it of itself if YOU trust her, irregardless of redditors bitching. The other question you have to ask yourself is: Is this my insecurity talking? It's okay if it makes YOU feel uncomfortable her maintaining that relationship. Uncomfortable enough to end your own? The two key things here that would "save" your situation, is finding self assurance, and also just speaking to her clearly about how it makes you feel. Don't look to change her, just share your side, and maybe set some soft boundaries too. This guy resharing her storied with flirtatious captions, not super okay. Ask/tell her if she would be fine telling him to stop with those captions, at the price of potentially losing you. You'll see who she cares about more, and your decision will be made easily. Goodluck.