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.