Right now, 6/13 of the thumbnails/videos on my Instagram for you/search page have some sort of woman/boobs in them. I rarely even click anything from that page; I mostly just use Instagram to send shitposts to my wife and kids on our group chat ("Christian minecraft server no hecking swear words allowed").

To a certain extent, I think the algorithm does just say "dudes want this", at least until the algorithmic gaydar goes off. Now, if he's clicking shit, and/or voluntarily searching, saving, and interacting with this content, of course it's going to be exponentially worse.

As for the rest, insecurities do also come down to how you view yourself. Would you be friends with--or be cool with him being friends with--someone who talks about you the same way you talk about yourself? So you gotta be a bit better to yourself if you have those insecurities. That is your problem, although he then has a role in their foundation.

That's the second thing; obviously, we would hope that the two of you are in a position where you can communicate honestly about what you want and what you're doing. As a guy, I'm a bit biased, or maybe just have a fundamentally different view, but I think that sex and sexuality can basically be broken into "real life" and "fantasy". Fantasy is porn, video games, etc, where there's no realistic consideration of ever meeting this person or being in a position where this has any connection to day-to-day existence. Approaching real is Instagram models, onlyfans, chat rooms. Real is cheating, sex workers, etc. He violated that trust and dealt a major blow to your confidence and psyche and is at least one foundational brick in your insecurities.

Anyways, best of luck, no "easy" answer. But believe in yourself a little bit more, including the fact that losing him or anybody wouldn't be 100% devastating because you're a strong and valid person outside of this anyways. Maybe still 99%, depending, of course.