You are obliged to confess KNOWN mortal sins by kind and number. That is it. Finito. If you are dealing with a sin where you doubt you considered what you were doing or you doubt you knew the gravity, etc., then you are not obliged to confess it. *
I don’t think a scrupulous person should go every week with a list of 10 things they suspect are mortal, but aren’t sure. If you are like that, go to one confessor regularly, let him get to know that you are like that, and follow his advice, not just for the one particular sin of that day, but on how to confess whenever you find yourself in knots. Do what he says.
Nick, if after making a good, honest confession, you later grow doubful about it somehow, that *maybe **you failed to say some sin or another, it is fine. You do not need to fuss. If you absolutely know you forgot to mention a mortal sin, then mention it at your next confession, but your confession was a good confession and you can go to communion.
I affirm this with my Baptismal Priesthood blessing.