When I started going to my gender therapist, I understood myself as being a "cross-dresser". That part was easy: all I had to do was look down at the skirt to know I was cross-dressing.
But I didn't understand then why the clothes (and forms) were so important to me, and I struggled a lot with the possibility that I was a "male" (in gender) who just happened to like (well, need) to wear the clothes. It took a fair number of months of my looking for "proof" that I was or was not "male" before I was ready to understand myself as transgender. One morning I didn't know if I was transgender, and then a few hours later I knew.
You could be "diagnosed" as being transgender any number of times without you knowing it for yourself. If (or when) you know it for yourself, getting a "diagnosis" will become mostly irrelevant to you, other than perhaps getting paperwork for bureaucratic purposes. (Or, in some cases, getting paperwork that you can show an SO or parent who need to see it in writing to be able to accept the situation.)
The diagnosis is an easier part. Accepting yourself can be rough for many of us. Figuring out what we are going to do about the situation can be more difficult still.
