Deep down inside" describes his true feelings, and the singer feels that she is the "perfect" girl for him. She is everything and the only thing he has ever wanted. This is why the "best years" of his life are the years he has spent with her.
In the beginning he didn't realize just how special she is. He didn't know what a great "team" they would be together. Now he sees that as a couple they can do lots of great things.
He feels so "grateful" (thankful) to have met her. Sometimes he can't believe how lucky he is that she loves him. The singer knows that his life would be very different if the two of them were not togerther. In fact he can't imagine them not being together, and hebpromises to ne with her "through the years" (forever) .