It's rather presumptuous to assume we don't think about this: I know a lot of trans people, both pre- and post-op, but none of us are walking into it with our eyes closed.Your question about ' how I know etc.' Perhaps you may just might like to think a little before making a decision that may create problems unforseen.
I've spent a very long time thinking about what's right for me but after 3.5 years I think I've had more than a reasonable time to contemplate my options, 2.5 of them at CX, yet I still don't have their blessing for God only knows what reason (if the clinicians there do have a reason, they're not telling). I don't think there's anything to be gained in dragging things out to the point where I'll die of old age before I get the chance to complete my transition on the very small chance I may be one of the people who regrets it.
I don't expect surgery will be a panacea, but I do expect it to go some way to resolving the severe physical dysphoria that continues to be a crippling influence on my life.