You have opened up how you feel in this forum, maybe you should do this with your girlfriend aswell....tell her exactly how you feel.....tell her why you haven't been yourself lately...don't feel that by being the 'confident' 'lead' in the relationship that you have to constantly be like that....every woman would tell you that there is nothing wrong with their man opening up.....heck i've cried in front of my girlfriend, we've been together 5 years this year and seen some up and downs...and with her moving away 2 years ago...and i'm moving even further away this year....its going to be even harder...just like you have it hard...
The only advice i have for you is to explain to her how you have felt that last couple days etc, how its affecting your sleep etc, I try the whole 'so what', i generally just use it when someone asks me if me and my girlfriend are going to last...i joke and say we may break up next week, but i laugh as i'm only kidding.....but deep down its hard to be emotionally drained by everything, especially when everything gets on top, and you end up taking the frustration out on the ones you love.....
The best things you can do is be positive, optimistic about your future, and tell her exactly how you have been feeling...
At the end of the day, you are both young enough to enjoy everything about life and love without having to commit just yet....commitment is hard....me being with my girlfriend for nearly 5 years and i still won't tell you we will def be together next year......because anything can change, we are all still young, with bright futures, and i'm just happy that at this precise time in life i have someone i can call and tell her everything about my day ...the ups and the downs.......life is too short......we all know that....be positive....speak to her...tell her everything...you won't regret it