Apology Letter For Forgetting
Dear {Recipient},
I'm really sorry for forgetting to {action that needed to be done} like you asked. I was planning to do it on {day of the week} but I've had a lot on my plate recently.
Please don't think I'm trying to get out of it, though. I'd still love to help you out. Would an extension be possible? I promise to get it done by {day of the week}. Many thanks for your patience and understanding.
Best,
{Your Name}