You are obliged, not merely encouraged, to confess it at your next confession, but you are not bound to have to rush to the next available confession on account of this. You can do this whenever you make your next regular or necessary confession, for as long as you still remember it. You can clarify by stating that it was a forgotten sin and that you’re mentioning it now.
This also does not mean you need to go back to your past and comb through everything for every possible mortal sin you may have committed and forgotten. This is a recipe for scrupulosity, and the Church does not want her children troubled with the past.
Also, my confessor made this clear for me: let’s say you confessed to a particular species of sin, say, fornication, and you’ve been a longtime fornicator, years on end. Then you repent and to the best of your ability, mention the approximate number of times you committed the sin, are absolved and at peace. THEN, you recall, you may have had a few more instances, say, fornication with people you’ve forgotten about (hey, could happen!).
In this case, you do NOT have to re-confess the forgotten instances, because you’ve confessed the same species of sin; only the number was off, but that’s not because of any malice or negligence on your part. The sins have been confessed and directly remitted in the Sacrament, and do not count as “forgotten” sins.