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?
It was absolved already. If it is a venial sin, you do not need to confess it. If it was a mortal sin, it was absolved, provided you at least implicitly intended to mention it if you had remembered it. What this means is that you confessed all known sins and would have confessed whatever others you committed had you recalled them. So if you now recall a mortal sin, mention it the next time you go to confession.