Yes, you must confess it at the next confession, providing you don’t forget again - been there, done that.
After returning from a life of Catholic indifference, I had to do a lot of learning and got schooled in the confessional.
Just kidding - he was good about it. Actually, I asked my confessor, who knew my closet was being cleaned out - a process of purgation that occurs during any kind of conversion - big or small.
If grave matter was involved, he told me I had to bring it up next time, but to be assured that it was already forgiven, provided it was not intentionally left out, and that I could continue to receive communion until that next confession or grave sin stops me. That is the key - it is forgiven when you forget, but as he put it, the Holy Spirit resurfaces these things for a reason.
I was remembering stuff over a 25-30 year period that was emerging gradually over about a 3-4 month time frame. Let me tell you, it weighed heavily anyway and the only way to remove the boulder was to cough it up in confession. Some of these things required counsel, which is one reason why I believe it is good that we do bring them up.