Re: should not have
#1 is contradictory. It's not possible to say "I had already done it" and "I should not have forgotten to do it" together. As you say "I should not have forgotten to do it" means "I forgot to do it and I regret it. I should have remembered".
Originally Posted by navi tasan
#2 is better though I wouldn't bother with the negative. I would end it "It was essential that I do it".
In BrE, we "fill in" forms, sometimes we "fill out" forms but we don't "fill forms".
Remember - correct capitalisation, punctuation and spacing make posts much easier to read.