Jealousy is often a diagnosis term for an underlying problem. In this case, your Dom is not making sure your need for his time is being filled, and bringing another in exacerbates that issue.

I would have no problem with poly situations if I were given my due time. But in a recent circumstance in my life, I have been shown by his lack of time, exactly where I stand in his world. Therefore... I have begun looking elsewhere.

Good luck with your situation.