My fiance [39/F] caught me [30/M] with porn and thinks I am cheating

She caught a screenshot from (goddamn) google photos that puts previews on everything, even folders, and was unhappy that I have porn (of the solo/webcam variety) on my phone. She's pissed that I would supposedly rather get off to "trashy women" than have sex with her, which isn't true...this does not interfere with our sex life. Worse...she thinks those videos were sent to me by other girls and that I'm running around behind her back.

I have never cheated in any of my relationships, and that includes even flirting with other women. I am a taken man. I am trying very hard to understand her point of view, but I'm struggling and I'm terrified that she is slowly melting down. What can I do to convince her I am not cheating?