AHHH, DOUBTS: You make for a horrible bedfellow!
As you can tell from reading here, doubting seems to often be par for the course - hugely for me for about a year.
Therapy really helped, treating my depression was a godsend, support group was cool, and finally beginning to dress and have time in my gender of preference drove the doubt away. Well, I'm human and have doubts now and then whether I want them or not - the difference is I deal with them without the awful feelings and paralysis.
You can too. It's part of coping with your transition. You will have doubts. When you do express them to people who can listen helpfully (like here, therapist, support group, etc.). Putting your doubts into conversation is the first step in robbing them of their power.
Take good care and stay in touch,
Rachel