The way to get past these insecurities is to change your frame of questioning. Instead of asking, "what if I'm not right/what if I change my mind," instead ask, "in the unlikely event that I'm wrong, am I willing to live with the consequences?"
I was pretty sure I wanted SRS. Tucking sucked. I hated having this one jarring out of place piece of my body. I hated standing naked in front of a mirror and seeing a woman everywhere except between my legs. I wanted the sexual experiences that can come with having a vulva. I was pretty sure I wanted SRS.
But I'm way too analytical for my own good. I also got caught in analysis paralysis, wondering, "what if I'm wrong?"
What allowed me to get past these thoughts was to stop seeking 100% certainty. 100% certainty is not possible in this world on any decision. Instead of seeking 100% certainty, aim for 95% certainty and a willingness to live with the consequences if you're wrong.
That's what got me over the hurdle for SRS. I was pretty sure I wanted it, and in the tiny chance I was wrong and later regretted it? Well in that case I would be in the exact same situation as our FTM brothers. Worst case scenario I have to go through what the other half of the trans population has to deal with. When framed that way, suddenly making the wrong choice on SRS doesn't seem like such a life-ending mistake.
I had SRS in 2013. Best decision I ever made. Zero regrets. Hell, my entire life as it now exists would have been impossible without it.