I have recently just under 2 months ago started seeing someone new. It was a surprise to us both as we have known each other since high school but never really thought of the other in that way.

I'm leaving at the end of this month for 6 months. Talk about wrong timing. He has said that he will wait for me, and come out and see me whilst I'm away. We've talked about it but mostly when drunk as he kind of shuts off when I try to talk about it sober. He's said he hates cheating, and only cheated on one girl when completely drunk and he broke up with her straight after because of the guilt. This should be reassuring but I still don't want to be that girl, because there is a chance he could do it again.
Just before we got together he had been seeing the most pretty girl, however she left for university in the September. I'm not quite sure what happened within their relationship, all I know is they were seeing each other for a while and that he waited for her whilst she was at uni but she didn't wait for him and so it ended.

When we first got together we decided that it was best to keep our relationship a secret. This was because we didn't want my ex to find out as he was sure to flip out at both of us. We had gone out one night as we have mutual friends and said girl was there. She was out at the same club and asked him if he was going home with her that night and he said no. Obviously she didn't know he was with me but surely this shows she still wants him?
They worked together and she will probably go back to work there when she comes back for the summer so they will most likely be in close contact whilst I'm away and I'm really scared he's going to leave me and go back to her. They keep in contact a couple of times a month on Facebook or via text but its always a brief “how are you” etc or for contact details of others and is always initiated by her. I also found a poem he wrote around September obviously about her which shows he did have real feelings for her, I just don't now if they are still there. He seems completely in to me, we are always together and he's very affectionate all the time. I've already met his father and he wants me to meet his mother. All the signs point to him really liking me but I have this horrible nagging feeling that I'm just borrowing him and he really belongs to her. I'm scared that when we sit there and listen to songs about leaving, that I'm thinking about him and he's thinking about when she left.

We are openly together now though but haven't changed status' on Facebook. I don't know if this is because I'm going away and we don't know how it will pan out or if he just doesn't want her to see it? Or for completely different reasons all together.

I'm already worried about how our relationship will last whilst I'm away, if he will wait or if it will just fizzle out because we haven't been together that long and we are only young. And now this girl quite frankly is making me feel really intimidated. She's prettier than me and more self assured and I feel like she would be better for him than I am. I REALLY like this guy. And I've not felt this way before. I know people say that and end up realising they have, but I also know I really mean it. I know I will wait for him whilst I'm away, I don't want to be with anybody but him.

This whole question is obviously highlighting my insecurities but I wanted to present all the facts so I could get the best possible advice in return. Any help will be GREATLY appreciated. Thank you.

Go do what you need to do for six months. If you both feel the same way about each other you can pick things up again when you return. You are only two months in to this and there is drama enough already.
Take a break while you're away and see how things are when you get back.
You may not feel the same way about him, nor he you. You may met someone else. Give each other space to see if what you have is real.