It's a daunting task but take it one step at a time.
Start by going through your password manager, checking each login. If it still works and you don't need it, look for a delete option on the website. If there isn't one, create a throwaway email address. Change your details on that website to be the throwaway email address, then change the password to something completely random. Don't save the password and then blitz the email address without keeping a record of it. If you have a bookmark for that site, delete it.