We are obliged to confess all “mortal sins” in number and kind (adultery 2x, murder 4x, lust 2x,etc) and if something changed the mortal sin…like it was your brother you murdered. Of course after a good examination we may not …after so many years…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…that is a sacrilege and one is not forgiven but has another sin)
Also we may not know the numbers …we do what we can…and if we do not know…we make an approximation…3-5x, 5-10 a month for the last 20 years, somewhere around 50 times, around 10x, or even if needed “many times”, “many many times” etc Depending on what we know.
As to venial sins they may be confessed or not confessed (though it is recommended to confess them…they can be forgiven in many ways). They need not of course be confessed in number.
If you doubt something was a sin…say it was doubtful. If you doubt it was a mortal sin say due to your lack of knowledge …well one can confess it --but note the doubt.
Remember Jesus loves you and wants to forgive you and give you always “true life” in him. He is the Good Shepherd!
If you forget a mortal sin …one is to confess it in the next confession.
jimmyakin.org/2006/09/a_reader_writes_1.html
Now…of course it can be the case that the sins of ones youth may not have been mortal …for one needs grave matter, full knowledge and complete consent…