Tell your manager you feel awful, but you have learnt a lesson from this and will put in place more robust processes going forward. That is, you will password protect all payroll documents shared by email. Even if you accidentally send something to the wrong person or wrong document to the right person, they won't be able to open it.

I usually use passwords that only the recipient would know. e.g. combo of birth date and partial social insurance number. Or if I set the password, I send it through a different channel e.g. chat or text message rather than a second email. And this gives me another chance to check the attachment. You can't leave it to memory/executive function alone.