How do I stop iMessages sending from my email address on my Mac?

Just the other day, messages I sent from my Mac using iMessage are persistently sending from my email address instead of my phone number (they're fine on my iPhone, and still sending from my number).

I've tried going to Messages > Preferences > Accounts to see what the issue is, but my phone number is no longer listed as an option under the "You can be read for messages at" section.

Does anyone know how to re-add my phone number here? Or why this problem may have occurred?