I will repost a general post on the subject of forgotten mortal sins – remember such is referring to mortal sins.
Honestly forgetting to confess mortal sins
-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)
jimmyakin.com/2007/03/specific_confes.html (Jimmy Akin of Catholic Answers).
More on forgotten mortal sins:
jimmyakin.com/2006/09/a_reader_writes_1.html
As to “doubtful mortal sins” (like I am doubtful if I gave complete consent to it) the general recommendation is that those of an ordinary conscience should confess them (noting they are doubtful) and especially those of a lax conscience (again noting the doubt). Though there is not a strict obligation to confess them.
Those who struggle with scruples are often rather recommended to not confess them directly (if they do they too would not it is doubtful).