No one I know makes 100 decisions and finds all 100 were right or good decisions. We all make mistakes. If you purchased a stock say for $10,000 and the value has gone down to $6,000, maybe you should consider selling it to recognize a capital loss.
If you filed your U.S. Individual Income Tax Return for 2016 and now you realize it is not correct, consider filing an Amended Return on form 1040X. With all the special rules and the difficulties in gathering all the tax information, it is not unusual for a return to not be quite correct.
First a little background. One of my grandfathers started a mortuary in Hardin, Montana just over a hundred years ago. It is still run by family, currently one of my cousins. My father had many occupations, including being a mortician.