if You have done a mortal sin without knowing you did it
That is a contradiction of terms.
What you are really asking, it seems, is “Can you lie to yourself?”
And that answer to that (without reference to “God thinking”) is, yes you can lie to yourself.
Then the next question is “Are you aware you are lying to yourself?”
And the answer to that is, unless you area pathological liar, yes, most certainly.
We all can exhibit a good deal of “mind chatter”, and if we take a moment to be self honest, we will admit it is mind chatter, simply playing solipsistic games with ourselves.
If someone has not reached the level of maturity to be honest with oneself, then one needs to speak with a priest or a counselor to come to terms with self-dishonesty. a=All of which is another way of saying, “it is time to buck up, buttercup”.
Hell is wide open for those who lie, particularly to themselves. And trusting yourself has nothing to do with it. Truth telling does.