We are never fully able to grasp the entirety of our decision to have SRS when we make the decision. There are things that our crystal balls will just not bring up on the screen. I fully agree with SummerBreeze in her sentiments. You will die to much of your prior life right there in the OR and you will grieve that death for several days after you come out of the anesthetic. Perhaps you have a foretaste of that approaching, and that is very good, it is proper and very normal.
If you seriously start having doubts, then it is a good idea to immediately get in touch with your gender therapist for a talk, not everyone needs SRS to live a good life, I was not one who could do without it, but they exist. I did have moments of wondering if I had made the right decision, even several days after the surgery, but they got fewer and fewer as the days went by.