it just gets easier with age and exposure honestly, by my mid-20s i stopped caring. i know the brain is supposed to be fully developed by 25 so i imagine maybe that has to do with it. of course, someone 18-21 is capable of distinguishing fantasy and reality too, but at that time in my life i definitely struggled more with believing my fantasies said something about me as a person (they don’t).
it also helped me to be around like minds or share my fantasies with other people who are interested in the same things, especially since i am asexual and this is the only way i really engage with my sexual interests at all. what actually finally broke down my last barrier was a fanfic i read that catered to my tastes so specifically i was just in awe. knowing that someone else posted that publicly without shame, and how much i enjoyed it, convinced me that sharing and posting my own very specific fantasies might appeal to somebody else in the same way.
so tl;dr time and exposure is what will eventually teach your brain that sexual fantasies are fine and harmless.
just continuously take stock of how you’re feeling and what is or isn’t working for you, and if you’re concerned about hypersexuality and how that may affect your propensity to be drawn to certain content, or if you feel it may cause you to seek real life risky or unsafe sexual encounters, i would suggest talking to a professional if possible.