What if I forgot to confess a sin and remembered it later? Was it absolved?
If I cannot recall if I mentioned a particular sin the last time I went to confession but recall it now, should I mention it in my next confession, or was it absolved already?
Yes it was already absolved provided you at least implicitly intended to mention it if you had remembered it. If its a venial sin you needn’t confess it, though the Church encourages everyone to confess venial sins too. As for mortal sins, you are forgiven as long as you intended to confess every sin you remembered while in the confessional. It is important also to do some recollection before going to confession to help you remember all your sins. So if you now recall a mortal sin, mention it the next time you go to confession.
I add and for everything I have done that displeased him.
If a person confesses and has his sins absolved by a priest, and then dies does he/she go straight to heaven, or does he/she still have to go through purgatory?
I say this after confessing – “for these and for all my other sins, which I cannot now remember , I am very sorry”
If you mean the person dies immediately as thy leave the confessional, that person would still go through purgatory because the temporal punishment (penance) for forgiven sins has not been completed.