Whenever you create a new account online, write down the email address it’s linked to and the password (or PIN number if applicable).
Include any other important usernames or reference numbers like a customer number or client ID. 
Either use an online notebook as default on your phone, a todo list (Microsoft) or just an old fashioned pen and paper.
When you go back to that account later and you’ve forgotten the password you’ll always have it jotted down to remind you.
Forget about being locked out of an account due to failed logins, because it will never happen again.
Why are we so lazy not to take another 10 seconds to record this information for later?