EDIT (if allowed): the person is my partner's daughter who I have a great relationship with, and has been talking to me since she saw what she apparently saw. She's a teenager and has told me in the past I "have a good heart". We've got a great bond. She saw a screen on the bus to school for a split second. I admire her for being loyal to her mom, but this is a real bad thing. It kills me she may think the same (even though we still chat).

I was scrolling reels on the bus with someone my Fiance trusts (more than me).

A dating app ad or dating site reel came up, one of those creepy AI or whatever ones that has your name to make it look like it could be you "meeting single women in your area" or whatever (because the app has your name? I don't even watch porn). This someone saw it, told my girlfriend, and now she thinks I'm having a secret online affair, looking at porn, or something.

We've been together 5yrs long-distance. We see each other about 3 times a year. I pay for all flight tickets, half the rent, groceries, bills, etc, each time I visit. I also pay for little things she needs when I'm in my own country (gas, takeout, etc)
She works hard and is constantly stressed at work, which affects our relationship. Her father has major health problems. Obviously, this has an effect on our relationship. I've tried to support her through the stress by just being patient, even if she's (probably unknowingly) doing something that's hurting my feelings, I stay silent and listen to her rants and try and empathise with her. There's not much I can do from 5000 miles away :( What I' saying is, I'm very serious about her and our relationship.

I've never cheated on a girl in my life. It's just never crossed my mind. I see myself as old-fashioned in that sense. I've devoted most of my 30s pursuing and being in a relationship with this girl, in order to hopefully settle down with her one day, which we've agreed we'll do when the time is right. I've had to keep my career static because I can't go for higher positions and then leave for 3 month a year. I've devoted my whole life to this girl.

My attraction to her hasn't changed (if anything, the opposite). All I've ever wanted is a family - I didn't come from a great one myself and believe in breaking toxic cycles.

I'm heartbroken at how sad she must feel that I've cheated on her, but also need to convince her whatever was seen for a split second on a crowded bus, then relayed to her by the person who saw it, was definitely not what she thinks it was.