From a reader:
If i forget to confess a mortal/grave sin in the confessional, when i fully intended to confess it before i walked in, is it still absolved?
If you truly forget to confess something, but otherwise make a good confession, then everything is forgiven. However, you ought to confess what you forget and then later remembered at an early opportunity.
Be diligent your your examination of conscience.
Always do your best. That’s all we can do.