I regretably made a bad decision
That can be a bad feeling. What bad decision did you make?
I cheated in a math test in high school
There is alot of pressure on kids to do well in school these days, but cheating on tests in school is not correct. Cheating will get you expelled.
It's fine, no one knew about it...until now.
It is wrong to cheat. It does not matter if others do find out because they always might, and they can even find out later on.