So she talks as though the kiss and confession is the betrayal, but that isn't true. It is every moment after that where the betrayal lies. Every lie, every expression, every single moment of pretending nothing happened is the betrayal. People fall in and out of love often, but pretending nothing happened to your SO is the problem.
She knew what would've happened if she went to the apartment well before she "considered" it. She was lost in the moment, but that moment has passed. If she doesn't feel regret for her actions, then she needs to end the deception instead. Only misery awaits her if she can't be honest with him.. Especially since she is being honest to herself.