Sada-e-sahw

I have a stressful problem with waswasah about "two prostrations of forgetfulness" like sometimes, I try to refrain from doing the "two prostrations of forgetfulness" when I feel it isn't obligatory to do it, sometimes I stop and sit about whether it's necessary or not - I probably do averagely two "two prostrations of forgetfulness" a day and that is a lot to me, and other times wondering whether I need to do the "two prostrations of forgetfulness" or not! Do I need to do the "two prostrations of forgetfulness" if I accidently elongate a letter or if I don't elongate when you should accidently? Question: What mistakes are acceptable for the salah to be valid without the obligation of doing "two prostrations of forgetfulness"?