My boyfriend is leaving me (advice long distance)

So my boyfriend that I love so much I cannot even put into words how much I love him is joining the army. I’m 15 and he is 17 (only one school year apart) he is joining when he’s 18 so we still have some time together but we are both really busy in school as I’m doing my GCSE. He has only just turned 17 but it makes me tear up (and cry when I’m alone) every time I think about it. I know it’s an amazing thing that he’s doing this and it will be so good for him but I don’t know how I will be able to cope. He won’t be able to see me when he’s doing his training or talk to me much which is about 6 weeks. After that he will get a little while off, then get put in a base and that could be anywhere, he told me it will be in London which isn’t too far from me but what if its not in London? And there’s girls in the army, he’s gorgeous. He’s going into the army for 4 year is what he’s thinking because he had this whole life plan. I know it sound silly because we are so young but we talk about our future together even though we haven’t been going out for that long. We spoke about moving In together and getting married and having a kid after the army. I’ve had boyfriends before but never like a serious relationship I’ve never been in love before and I always used to think when I was going out with other people what’s the point of actually going out coz I don’t see myself being with you for long. But this boy, he’s just amazing I’m head over hills in love with him. I feel like my heart is being ripped out of my chest thinking about him going. So what I really asking is help for a long distance relationship x