Sounds interesting. (Better than Drugs)
How about this, My character is a Military Policewoman returned from Iraq due to the drawdown of forces The is a tall, athletic woman - muscular for a woman, without being muscle bound). Perhaps with a mild touch of PTSD. The strong-but-wounded type. You want your character to be domme, fine. She doesn't exactly think she has sins to atone for, but letting go, letting another take control of her so she doesn't have to worry about things or make decisions for a while is very attractive to her, though she may not consciously know that.
You said you might switch, she could be stronger, able to take control when the mood strikes her, if it does.
Your character is her therapy, and much much better than what the US Army provides.
I see her as tomboy femme/soft butch/hard butch. Your choice!