Just write in the letter how you feel about yourself. No need to be grey about it. State the facts as you feel them. Choices are never black or white. They need to be adapted to your particular situation.
It is absolutely critical that you are being referred to a psychologist who has experience with transgender people. You don't need somebody who has his/her own issues and believes that you should be cured of your 'mental disorder'.
Second, you must ensure that you feel totally comfortable talking with that psychologist. If you feel constrained, you are wasting your time. If the psychologist is skilled, they will make you feel comfortable in session 1.
Hope this helps.