I dumped my fiancee to date a 10 and I have zero regrets
Do you feel that you are a ten too?
I'm not a ten. I have all the features of an average guy
It's okay to leave a relationship if it's no longer making you feel happy, and you shouldn't regret decisions that were made on that basis. However, leaving a relationship with one person for another, just based on appearance, is very shallow and will not lead to a very fulfilling relationship in the long-term.
I'm not dating her in the long-term. We are having casual sex, it's still very fulfilling.
As long as you both are on the same page about the relationship. As shallow as it seems, maybe it will work out for the both of you.