You are an amazing woman, Tess. But it's hard to see (Hard for her to
understand it because someone has too much power over her?) when
you're stuck in the dark shadow of an even darker man. Evil man?
The images overlap. It's quite common to say that A's brilliance is obscurred (OR outshone) by their companion, B.

The moon can be obscurred by dark clouds; or when the sun comes up, sometimes we no longer notice the moon simply because the sun is so much brighter. The moon is no less beautiful, but it's "hard to see."

You might say that the sun overpowers the moon, but you wouldn t say that the sun has power over the moon, in the sense of a controlling influence.

Robert Shuman's wife Clara was a fine pianist, composer, and critic in her own right, but you might say she lived her life overshadowed by her famous husband. (I exaggerate.)

So anyway, I'd say this amounts to a mixed metaphor, because a "dark shadow" has a different implication than an ordinary shadow. But it's simply used here to transition us to the dark, evil man, as you suggest.

But I repeat, I don't think we're talking about his power over her as suppressing her amazing qualities.
You're an amazing woman. But it's hard to see.

In this cas it's hard to see that she's an amazing woman probably because she doesn't show it.

In the second sentence...I have the idea that sometimes you are in a bad situations with an even worse person, a person worse than the bad situation.
Site Hint: Check out our list of pronunciation videos.
 Avangi's reply was promoted to an answer.
Thanks, Avangi!