If one confesses, receives penance, forgets to perform penance, commits mortal sin, remembers forgotten penance before next confession- is it too late to perform penance?
I don’t believe it is too late.
If you “forget” you cannot have the requirement of “full knowledge” for something to be a mortal sin. No sin has been committed so long as when you remember you preform the penance, or intend to complete the penance as soon as possible.
There’s no time limit or expiration date on penance.