If you are using a password manager, start by searching for every record with your gmail address. Make a list. Every day, go to the next entry on the list and change your email with that app or service.

Of course, set up gmail to forward messages to your new address and filter them into a folder. Once you have changed all the services you know about, watch for emails coming to the gmail folder, looking for more services that need to be updated. Eventually the only thing arriving in the folder is spam and you can just route it all into the garbage.