When I was accepted to my top choice master's program, I absolutely felt this awe at the situation. It didn't feel real given that I knew qualified individuals (whom I ranked as better qualified than myself) that had been rejected. After visiting, I decided I did not care why they wanted me; I wanted to be there. I think you just have to take some time and let it sink in. Visit (if you have time) and be sure the program is everything you have created it to be in your head. Otherwise, get excited! It's your top choice! The process is so stressful and frustrating that it seems only natural to be left a little jarred when it is finally over. Those letters are seriously anti-climactic!