In a nutshell;
1. You want the position
2. Since your previous boss is a bitch, I assume your job responsibilities would have been quite restricted and mundane.
3. Just say you wanted to widen your reach in the industry and felt that you could contribute to that new company. As well as develop yourself by experiencing a different management and style of operations.
4. Now you want to come back is because you feel you can contribute to the company in the new capacity and develop your career and grow with the company.
5. You like working with the old company and had good relations with collegues (i assume)
6. As your collegues to put in good words for you.
7. Up to you to convince them with your knowledge and net work.
8. Never talk bad about current or old company.
9. Position requires tactfulness and diplomacy, display that.
Other bros pls critic
previous sales, now the position is sort of like team/division manager so taking charge of both sales n non sales staffs