How can I protect the email account that unlocks everything?

Your email is quietly the most important account you own. Almost every other account - banking, shopping, social media - can be reset through a "forgot password" link sent to it. Get into someone's email and you can walk into everything else, which is exactly why it is the one account worth protecting hardest.

Lock it down

Give it a long, unique password used nowhere else. See how can I create strong passwords I can actually manage?
Turn on two-factor authentication, ideally with an app or a hardware key rather than text-message codes.
Check your recovery options are current and yours - the recovery phone and backup email should be ones only you control.

Watch for quiet tampering

Look for forwarding rules and filters you did not set. A common move after a break-in is a hidden rule that quietly copies your mail - including password resets - to an attacker.
Review recent sign-in activity if your provider shows it, and sign out of any devices or sessions you do not recognise.

If it is ever compromised

Change the password immediately from a device you trust, and turn on two-factor authentication.
Remove any forwarding rules, filters, or recovery details you did not add.
Check the important accounts tied to that email for password resets you did not request.

If you only do one thing

Give your email a unique password and turn on two-factor authentication. It is the lock that protects every other lock.