How long have you two been together? What was your relationship status when the flirting was going on? You said that you find her trsutworthy, but do you trust her? Do you trust that she will never do this again? If yes, then forget this entire incident, forgive her and move on. If you have doubts about trusting her after this, if even a tiny part of you is curious if she will ever do this again then you will have to explore this a little bit further within yourself. Because if you won't find it in yourself to trust her, you will light a long fuse and beginning of the end will ensue.

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My Demon revokes any prayer
He's grown contempt for love and hope
He betrays trust, twists truth and fair
Indifference is his way to cope
Engulfing sound of sensations
He quells with voices of despair
And muse of short lived inspirations
Flees at the sight of his cold stare
~Moy Demon - Mihayeel Lermontov~