hey, have you got the message yet that it's not something you need to panic over?!! Calm down and let yourself off the hook.
I only remembered last week that Alfie needed worming because I saw a worm in his poo!
<faints> (the vet had said worm every 3-6 months, and it was closer to 6 than 3 - I should have wormed him closer to 3 because he's always snuffling in, and eating, stuff he shouldn't).
It happens, and it's not a big deal (honest!) - though I confess I was mortified when I saw the worm.
To save you from beating yourself up over it again, when you do worm him this time, mark it on the calendar and then mark ahead to the next time he needs doing, as a reminder.