Hi everybody! This is my first post here (and I really hope I chose the right section for my following problem).
I'm not from Australia, however, after a brief google research this forum seemed like a good idea.

Long story short: I'm in real need of an objective advice in regard to the following problematic working issue:

I work for about 3 years in a company where the boss (he) is romantically involved with a coworker (she).

My boss grants me trust in handling my tasks, appreciates the work I do (he is pretty much a good manager). His "secret" relationship with this woman is the problem: due to this, she can be a "dictator" (shout, treat the other coworkers like trash ..., makes people cry/ full of anxiety).

Today, her "being her" almost made me quit. I left earlier (excusing myself that I wasn't feeling good) just to calm myself and not say in her face the truth.

My problems:
1. Every time I want to complain to my boss about how she behaves he says "That's the way she is... ", or avoids the subject. She managed to convince him to fire two other colleagues that "undermined" her ...
2. I like what I do and my colleagues (her ... less) but I can't go on working with her.

So, I can't expect a solution from my boss nor continuing to ignore her behavior.

Should I write my resignation and "thank my boss and colleagues for the great period spent there?"

It helped a lot. Thanks.