Firstly, you're joining the Army, stop being a fricking wuss. I wasted what should have been the best years of my career moping over a lass and, listen carefully, if the relationship had been as strong as I thought it was - it would have lasted, not ended in a nasty explosion of shouting, tears and thrown kitchen ware.
Yes you want this to last, that's normal, but you are about to meet two feck off great brick walls that both are you are going to have to deal with:
1) When you are in, there are 3 people in your relationships with ANY other human being - you, them and the Army. The Army wins every time. That stands for girfriends, mates, family, pets, favourite wnaking sock, the works. You and the people in your life had better deal with that or you need to find another job. By the way, the real kicker, especially when you're young, is that the Army always wins because deep down you want it to. Running around the cuds doing the job is more fun than being someone's boyfriend/husband/fcukbuddy. That changes when you get older, but don't hamstring your ability to enjoy the first, responsibility free years of your career. You need to be able to go anywhere, do anything, volunteer for courses and jollies and make the most of being a young soldier without worrying about a lass back home. Cruel but true, trust me.
2) People change. When I say that I mean EVERYONE changes. You're about to go through the single biggest change of your life so far. When you get to full screw look in the mirror and remember where you where right now, you'll see.
That goes for your lass too, she has a lot of things to learn about herself and her own life - what she wants from it and where she wants to take it.
So, way forward? Same with everything else - sit down, talk about it and be honest. let her know that the bloke who you'll be in 6 months is going to be drastically different to who you are now. You'll have COMPLETELY different goals and motivations. That doesn't mean that you won't want her around, but it means you'll have to both work a feck of a lot harder than her mates have to with their local boyfriends.
She also needs to know that no matter how hard she tries, if you're serious about your job (and by chr1st the Paras will expect you to be) she will come second. At least to begin with.
Don't burn your bridges just yet though, because it's a tough old course and you might fail. Going back home having come off the log, just to see your ex lass doing some civvy is a lot to take. But then you deserve it for coming off the log.