This will be harsh. You have been warned.
Maybe you're having problems relaxing during sex because you're having meaningless sex with guys who you have no relationship with and no connection to.
Here's an idea. Make friends. Male friends. Female friends. Old, young, middle. At the university, and not. Start to make connections with your fellow human beings instead of throwing yourself into bed with semi-random semi-strangers, hoping for a mercy ****.
In the meantime, learn how to pleasure yourself if you don't know already. Take a look at the Savage Love column (https://www.washingtoncitypaper.com/columns/savage-love) as there may be something in there and it would be a lot more reliable than just Googling and hoping for the best.
When you know how to please yourself, you're less likely to be sexually frustrated, you'll be better at communicating your needs to others, and hopefully you'll be more selective when it comes to your future sex partners.