Confused

I dont quite know what is wrong. I guess the best place to start is at the begginning.

First off, I came to this site to help, not to actually seek help, so forgive me if this seems a bit held back. A very close friend of mine, I just found out, has hidden something from me. Im hurt over that, but I understand why he wouldnt tell me. He doesnt want to look bad in my eyes. I dont understand why I hurt this badly though. It has almost nothing to do with this, it just seemed to trigger it. I confused and I think just laying things out will help me think. So he and I both decided we would be abstinate before we ever met, and that was one thing that brought us together. Then tonight I found out that he slept with another friend of mine a whle ago. Im not dissappointed in him, I just wish he felt like he couldve told me. I guess it just didnt work out that way. But to find out like this. It was so sudden, the girl who told me (the one he slept with) thought he wouldve told me. Im just shocked. I dont understand quite why I feel this strongly. It isnt really anything that I should be worrying about. I think Im afraid of losing him. Ive always wanted a little brother, and this past year as Ive gotten to know him, he has filled that void. Next year I leave for college and he's a year behind me. He wants to go into the military (good luck finding me there). I just cant stand the thought of someone this close to me so soon, and I think this might be the start of it, and it scares me, it really does. I know we all make our good byes someday, I just hope it isnt today.