Don't make the same mistake again, get a domain so that you can keep using the same address when switching between providers. Then set up GMail to forward e-mail to your new address. Then slowly update the E-mail address in your account. You could even set up a label that gets attached to e-mails that arrived through your GMail address. In that way, you can easily see the stuff that still needs to be updated.

Untangling yourself from Google (or Apple, which is similarly hard), doesn't have to be all at once. Break it up in small steps that feel like individual wins.

One more note about using your own domain: avoid provider-specifict features like subdomain addressing (made it more work for me to move off Fastmail).