It might be possible for Nabble to recover the situation but you would need to post the URL of your forum.
If the email address you are using is one from Yahoo, Gmail, or the like, then it might be possible to see if the wrong address is still available. Then you could login with the new address and change it to the one you want to use and then abandon the new address.
I'm not absolutely sure how things work, but it is also possible that if the wrong email address has not been validated, then Nabble will eventually purge the address (and associated username) from its database. It might take a while for that to occur, but you might find your preferred username becomes available again in due course.
However, I assume it is a brand new forum with very little customisation and few, if any, posts on it. In that case it might be just as easy to start a new forum and use your preferred username on that one.
I would always advise setting up a second user. That will also need a different username and different email address, of course.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.