tampons help!

I can't use a tampon nor was I ever able to.  I'm 18 and been having my period since I was 13. The thought of using one makes me soo nervous. When I eventually try to I don't know if it actually hurts or it's me freaking myself out. So I was never able to use one.  I am sexually actively so it not like I'm not able to use one. Any advice?! I want to use one to swim