Don't beat yourself up. You didn't forget Lily and she knows that at the Bridge she is still missing her mummy as much as you miss her.
You haven't forgotten her, you have forgotten to look at a number on a calendar. I bet you still thought of her on that day. Forgetting a date is nothing, remembering her is everything.
I am terrible with dates, I forget everything birthdays and all, it happens. It's life.
I hope you are still having fun at the Bridge Lily.