First, don’ freak out. You know them and intend on making an act of contrition. Good! As to how terrible some of them are, please allow Father to discern that.
As to confession, give the number and type of sin. Take the list with you, as nervousness often makes us forget. No details unless Father asks so as to tailor some spiritual guidance for you.
And, Father has heard much worse.
EDIT: When you finish your confession, it is always good to ask: “For these and any sins which I may have forgotten”, as that will cover those that may pop into your head the next day.
This Sacrament has the intention of restoring your right relationship with God. He will forgive everything, if you are contrite - even if you don’t remember them, He does and will destroy them.
He wants you to be at peace.