I just came back from confession, and realized that I forgot to tell about some venial sins. Is it necessary for me to mention them at my next confession?
No. Venial sins need not be confessed at all – although the Church does highly recommend the sacramental confession of venial sins – and so forgotten venial sins do not need to be brought up in your next confession. Only if you remember a forgotten mortal sin do you need to confess it at your next confession.
For the future, please remember that reception of the Eucharist is also effective for the forgiveness of venial sins. If you commit a future venial sin, you can obtain forgiveness for it through Communion.