I'm worried my girlfriend will cheat on me while I'm away in the marines?

I've always wanted to sign up for the marines, and I want to do it before I'm too old and they won't accept me. As of now, I'm 19 and so is my girlfriend and I'm scared to leave for so long because I don't want her to cheat on me or leave me. She's never cheated on me before so I don't really have a legit reason for thinking she's gonna cheat on me but the thought still lingers in my mind. I know I won't cheat on her, I'm 100% committed to our relationship and she says she is too but I'm still weary. How can I get over my fear?