Thank Susan for this amazing forum. The varied array of responses from trans people on trans issues found in this and every thread on this website - is an invaluable resource, and I would suggest not taking any of these people's experience lightly.
Of course we ought not urge you toward our destination. You can only ever know if transition is right for you. But understand that should you choose to transition, it will be distinct and unique for you. None of us go to the same place, all of our transitions are custom made to fulfill a personal inner peace.
This doesnt mean everyone can transition happily. It certainly doesn't mean you Should. You have carried this with you for a lifetime. How can you unburden yourself?
Isolate your sexual fantasy to be a woman. It is far too risky to consider total life alteration based on a sexuality. Especially because the process to transform into your sexual fantasy will likely redefine sexuality as you know it today - and the fantasy you set out to realize will be gone.
There are no rules. You are doing the right thing asking questions, you obviously spend a lot of time on this issue.
HRT can be diagnostic. If medical transition is right for you, the changes HRT brings will give you a sense of peace and joy. If they do the opposite...that being said, I don't believe your 20 days on HRT should be seen as evidence against your transsexualism necessarily. Hormones are scary. They are a game changer, and should be taken with incredible caution and clarity. You may need hormones but perhaps were not ready.
This is a long and slow journey. If you don't know if you want or need to transition now, it's OK. There is no rush. I know you want to figure this out before you're 40, but if you remain present and active on this area of your life..you will.
Get back on therapy.
Devise a plan to find out what this is all about: go out dressed, try dressing at a time you're not Horny! If that means jerking off and then getting your dress on, do it.
Keep talking. Reading is great but nothing beats discussion like this.
Be patient....everyone on here will tell you that - listen.
Also! Consider working with your doctor and therapist on this idea - taking Testosterone blockers alone. Taking only T blockers for some time will cause the sexual side effects of HRT to begin. In this way, you would see if your sexual urges were the driving force behind it all, without growing breasts (probably) in the process. Of course being only on tblockers has its risks, and is obviously only possible in safety under care of a physician.
Join a support group so you can build relationships with trans women. I did this immediately because I wanted to see if I related to them at all. I learned that we are all different, but I did find a commonality that is hard to always define.
Be gentle on yourself. HRT is something that works better the younger you are, but there is no major difference between starting at 21 or 25. Take your time. X