The best way to not let second-guessing get out of hand after your primary treatment is to take the time to get consults on the rest of the options you have available.
My surgeon would not even talk about booking a date until I reviewed the other opinions I had gotten. He required me to talk to a radiation oncologist and discuss the case with MD Anderson, as I was thinking seeds or Proton as options.
It doesn't mean that you have to get a degree in PCa as some here have done , but you need to be comfortable with why you made your decision, as that desire to second-guess will continue to try to come out.