Hello vansanityyyy,
You mention in the notes of your post you also have an iPhone with iOS 15. If you use the continuity feature it's recommended you check your settings on both your Mac and your iPhone for Messages. On your Mac the use the following to check the "start new conversations from" setting in step four:
"Turn your phone number on or off for your Mac
After you set up your phone number, you can turn it on or off for Messages and FaceTime on your Mac.
- Open Messages or FaceTime. If asked, sign in with the Apple ID that you're using with Messages and FaceTime on your iPhone.
- For Messages, choose Messages > Settings, then click iMessage. For FaceTime, choose FaceTime > Preferences, then click General.
- Turn your phone number on or off:
4. If you want recipients to see your phone number and not your Apple ID, select your phone number from the “Start new conversations from” or ”Start new calls from” pop-up menu.
"
You can also use the steps above to check the current account used with iMessage on your Mac. If needed, all the information above can be found in the article here: Add or remove your phone number in Messages or FaceTime
There's additional information and helpful steps for checking your Message settings in the guide here: Set up Messages on Mac
If for any reason you're unable to view or change the options in the settings, check your restrictions with the help of the guide here: Change Content Restrictions settings in Screen Time on Mac
Hope the information helps.
Cheers!