Yes, for the love of god, make sure you bag up. If she for one second thinks that you might be about to sign up, she might well go for the oldest trick in the book of getting knocked up.At 19 I too was in love, desperately, joined the TA because it was something I wanted/needed to do and she didn't want to see me in the Regulars. Also I was in the Customs and Excise and only came home at weekends.
It wasn't nice to find the bed still warm from other blokes, despite her telling me I was the only one for her. Destroyed me.
The point it, while you are young and in love, you know what you want, does she? Will your relationship change over time? Of course it will.
Will you stay together? Who knows? Perhaps you stay out because you respect her wishes, then for some reason you split. Perhaps she changed or wanted something else and moved on. Where will you be then?
What if she finds a job elsewhere? Will you be expected to follow her, or will her priorities change?
If you feel the Army is what you really want to do then go for it. If you feel you want her more then choose.
But if you go for the Army then a big beware, don't get her pregnant, there is more than one wench who has used that trick to keep a man at home or close by.
And good luck
DO NOT trust her if she says she's on the pill.