Where does "Look yourself in the mirror" come from?

Etymology-wise, this may come from a conflation of two older/clearer phrases:

To look someone in the eyes (or in the face, or eye): as Rhodri says, this means to make eye contact with someone, and is considered a sign of sincerity, confidence, and absence of shame or guilt. This is a somewhat peculiar idiomatic phrasing — it seems to use “look” as a transitive verb, which isn’t usually correct — but it’s fairly old and widespread.

To look at someone/something in the mirror: this is pretty clear — it’s a perfectly standard use of to look.

So then look myself in the eye and look at myself in the mirror get conflated as look myself in the mirror, which doesn’t quite fit either of those patterns, nor the standard usage of look. As Rhodri said, it certainly has the same meaning as look myself in the eye. Grammatically speaking, I must admit it felt wrong to me when I first read it; but googling around, it’s that it’s very widespread. Is it perhaps new, or is this just recency illusion on my part?

She wants to be able to face herself. Also known as "I want to be able to sleep at night" or "I want to be able to get up in the morning". She doesn't want something negative hanging over her. It's a decision she doesn't like, doesn't want to take and in the meantime is hard for her to have waiting.