If I could send past me a letter it would say:
Dear Sir:
Yes, I said Sir. And you know it. Wake up and smell the coffee and get yourself to a gender therapist. If you wait another 20 years, you will go through that much more pain trying to figure yourself out. You will develop a more female figure and hate it. You will gain weight, which will make your figure more female. You will have to have a surgery that is more invasive than if you had it now. You will ruin relationships without knowing why and get into the wrong kind of relationships.
Get on with it. Transitioning now will save you a lot of trouble.
Love,
Dennis (future you)