All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
Forgetting and making mistakes are normal characteristics of humans; it is for this reason that Allaah does not hold us responsible for things we commit by mistake and those which we forget to do. Allaah Says (what means): {Allaah does not burden a soul beyond its scope.}[Quran 2:286]. Furthermore, it is confirmed that the Prophet said: “Allaah will not hold my nation to account for things they do by mistake or they forget to do, and for things which they are forced to do.”
Therefore, you are not sinful for the loss of things resulted from your forgetfulness.
For more benefit on treating forgetfulness, please refer to Fatwa 90456.