Honestly forgetting to confess the number of mortal sins
(venial sins need not be even confessed necessarily -though very recommended --at least some…let alone numbers given)
-is well –
forgetting to do so. Hiding such is a different story. Assuming one was contrite and amended and intending to confess all mortal sins --and just forgot to say some —
*
they are absolved indirectly. *
One is to confess then in the next confession. Of course ones memory could honestly slip again despite ones intent etc to remember.
Yes one is to confess all forgotten post -baptismal mortal sins (well that is those one remembers!..sometimes we may forget something forever…). So if one remembers say that one murdered ones 5th grade teacher…and that it was mortal (not only grave matter but full knowledge and deliberate consent) and not confessed --one is to confess it.
(Now I will note for some out there – who struggle with scruples – they may be in a different boat in some cases. For some seek to confess all sorts of un-needful things --they need a regular confessor who can guide them)
One does not have to identify the commandment --but rather for mortal sins one needs to confess the species (even if one does not use the technical name)
jimmyakin.com/2007/03/specific_confes.html (Jimmy Akin of Catholic Answers).
As to forgotten mortal sins:
jimmyakin.com/2006/09/a_reader_writes_1.html
As to “grave sin” “mortal sin” and “serious sin” they are different terms for the
same sin. All are synonyms. Some can get confused there…but the terms are used interchangably by the Church. So when Canon Law states one must confess all grave sins --well what is meant is that which was committed (grave matter, full knowledge and deliberate consent). Hence all mortal sins (grave sins, serious sins -they all mean the same with a bit different emphasis) are to be confessed.
One can address this difficulty with a Priest in confession – and remedy what may be missing etc etc. You could make an appointment and perhaps (perhaps I say not must) make some notes --perhaps in code --to refer to and then destroy the list.