I'm sorry to try to drag this back to the topic at hand, but I think that certainty comes over time and with experience.
For me, the starting point was to try to ease into dealing with my lifelong gender issues. I talked to a therapist. I went to a couple of events presenting myself as a woman (best as I could). I talked to friends. I read as much as I could, including on this forum.
With each step I wanted to take the next step. But you may decide that you don't. Each of our paths is different.
As I got deeper and deeper into transition, I became more certain. But in the beginning I wasn't. I just proceeded to the next thing, saw how I felt, and then decided whether to take the next step.
Now that I have been full time for over 9 months and am out to everyone who knows me, I am absolutely certain I am doing the right thing. There is not a single doubt in my mind. And this isn't: "I'm afraid to admit doubt." This is: "I
know."
My point, though, is there is a lot to cover between "am I?" and "I know". Take it a little at a time. Explore.
And good luck in your explorations.

- Kate