One is obliged to confess all mortal sins in number and kind (and circumstance that changes the kind – like it was your brother you murdered! or the gold cup you stole was from the Church (hence sacrilege))
Venial sins are recommended to be confessed. They can also be forgiven in many other ways.
Of course after a good examination we may not …remember everything…that is ok…we do what we can…God forgives the forgotten ones too (we need of course to be sorry for all our mortal sins…and amended)…then if we later remember one we forgot…we are to confess it in the next confession after we remember it. (this is different from if we intentionally hide a mortal sin…)
If something is doubtful one notes it is doubtful in some way. While one is not strictly obliged to confess doubtful mortal sins – it is normally recommended to (though those with scruples may be recommended not to).
Also we may not know the numbers …we do what can …and if we do not know…we make an approximation according to our knowledge from our examinination …3-5x, 5-10 a month for the last 20 years, somewhere around 50 times, around 10x, or it was several times definately more than 10 …it may be that one has to say: “a few times”, “several times” “many times” “many many times” etc
Of course one needs contrition and a firm purpose of amendment.
Here from the Catechism:
scborromeo.org/ccc/p2s2c2a4.htm#VII
Here is a guide sold by Catholic Answers:
shop.catholic.com/product.php?productid=137&cat=25&page=1
From Jimmy Akin of Catholic Answers:
jimmyakin.com/2007/03/specific_confes.html
The Priest has heard it all before…just confess behind the screen and be matter a fact --do not worry about this …he has likely heard much worse.
Jesus is the
Good Shepherd …he loves you --in him is * true life*
(and thus in confession is true life!)