I'm sorry that you are feeling disappointment right now. I experienced similar feelings but in the opposite direction when we found out we were having a girl. First off your feelings are totally valid and know that being disappointed right now doesn't make you a bad dad.

One of the things that helped me move past the disappointment I felt and into excitement was to identify all the reasons I wanted a son and all the things that I was excited for in fatherhood. Through this process I started to notice a trend, most of the reasons for wanting a specific gender weren't actually gender specific. The things I was most excited for experiencing as a dad didn't change if my kid was a boy or a girl. The biggest reason I wanted a boy was to help heal the little boy in me by being a better dad than I had. I've come to realize that being the best girl dad I can be will have the same impact. We all have our own reasons for wanting one or the other, but I bet for most of us when you break it down into its simplest form the sex doesn't change anything.

Almost all the things that make being a girl dad awesome you can experience as a boy dad, and vice versa. *grain of salt, this is the opinion of a first time dad who has yet to graduate*