Military Man

My SO(20yrs old) & I(18yrs old) are both in college and are extremely happy, we've only been dating for about 5mon but it's been the most amazing 5mon. This is easily the best relationship I've ever had. He recently told me he's considering joining the marines. If he joins he's going to be gone for 13wks for Basic Training and anywhere between 2-6months for additional training after that. After training he gets his first duty station and it's no guarentee he'll be close or even accessible. He wants to make the relationship work because he says he's in love and he doesn't want to lose me. I was in a similar situation in high school and I got cheated on within months of separation so I'm extremely insecure about the whole thing. My SO isn't the type to cheat but I'm scared that if we don't take a break now that he might find someone else and leave me. I feel like I'm being so selfish but I just don't want to go through the trauma of another heartbreak considering that I only recently got over the first one. Does anyone have a similar story or any advice?