A matching domain exists in my account

So if I understand correctly, you bought mydomain.com from CX but that was put on hold (for some reason or other) and you never paid for it.

After that you created a hosted zone in Route 53 called mydomain.com (which you're allowed to, and doesn't mean you've taken ownership of the internet domain of that name).

Then you noticed that CX hasn't taken payment, and it sounds like they cancelled that transaction without it going through as mydomain.com was still available for you to register Route 53 (which you did).

At this point Route 53 will automatically try to create a hosted zone called mydomain.com but it can't because there's already one there with that name.

I think you should be fine, you're basically doing this but in a different order https://docs.aws.amazon.com/Route53/latest/DeveloperGuide/domain-replace-hosted-zone.html

Create an A-record for mydomain.com and point it to your WordPress site, and then add www.mydomain.com as a CNAME. Let it propagate and see how you go.

In terms of your last comment that you don't want to "lose everything I worked on" how would that happen - have you paid somebody to host your Wordpress site, and that relationship is dependent on you registering mydomain.com through them?

Yes, in this case you should edit the name servers on the registered domain to point back to Lightsail DNS zone name servers. To do that:

1. Go to "Domains & DNS" tab in the Lightsail console.
2. Click on your registered domain. In the Details tab, click on "Edit name servers" option.
3. At the bottom of the edit menu click on the option that says "Use Lightsail name servers".
4. You'll see a popup asking to confirm the replace name server operation. Select "Yes, replace".
5. Click on "Save".