Just add the new key on a new line in the same file.

I can paste anything into that shell. I am unable to ssh in from my terminal. When did this become such hassle?

Thanks. It was that simple.

Hello there,

You can check our article on How to Upload an SSH Public Key to an Existing Droplet

https://www.digitalocean.com/docs/droplets/how-to/add-ssh-keys/to-existing-droplet/

I will recommend you to edit the .ssh/authorized_keys via ssh using an ssh-client and not using the DigitalOcean console.

Hope that this helps!
Regards,
Alex