Realize that you'll never hit 100% certainty until you come out and actually try it. I was like 99% certain when I finally worked up the courage, but that 1% was still really bothering. What got me over the edge was accepting that I need to take a leap of faith in coming out. I needed to accept my doubts and push past them, or I'd be paralyzed by fear forever.

I was at the lowest point in my life and had nothing to lose, which made that leap easier. However, I recommend against letting it get to that point. Coming out was the best decision I ever made; it made me finally see the world in color and live as an actual person for the first time. It'd be almost impossible to convince me that I was cis at this point, even if you had full control of my 5 senses and were poking around in my brain.