Old story came out and i don't know if i have to confess

Hi, im dating with a girl since january and we keep it up until now, i'd like to say we are enganged, even still i'm not letting go.

In the quarantine, like 2 months that i was dating with the first girl, i start talking with another girl (a friend of mine) about a game that we both play, but things changed quickly in 3 week (so 3 months) where she said she was interested in me, and i said that i was too when i saw her the first time.
She had a boyfriend, since 1 year and half, but we ended up talking for like a week (maybe 9 days) with explicit sexual messages, pics, and video where i have a climax, putting down my pic with the first girl when i was doing that....
after those 9 days I couldn't stand it anymore, and i said to her that i couldn't do that to my girl, and maybe we can try if one day we both will be single.

She cried, saying that she was sad, but i was in my position, then things still was little hot, but then everything became calm. We still talked, but as friends, and honestly im not even attracted to her anymore, or maybe and i wasn't much attracted even that time.

the trouble started the another day when her boyfriend found out that there was something. He saw an old screenshot of a chat group where a random dude shows his "member".
That was because me and that girl was in those weird chat where people share nudes and things like that. But that was like about 15 minutes. I was inserted in that chat, i see that it was an illegal chat, i shared the screenshot to her saying "wtf is wrong with these people, im gonna go out and report it" and then there is the end.

What's ridicolous it is that her boyfriend starting suspecting about EVERYTHING happened in quarantine, blaming and leaving her. She told me yesterday how he figured out how we was touching ourself while talking and other things like he already knew everything; he said that she is a b*tch and i'm disgusting. She can't lie, and in some way she confirmed almost everything -_- .

Now now, i lived well until now, since i was always in the mood of "i was just dating since a couple of months with the first one" (and one was of quarantine), "nothing was official" and "i didn't touch anybody, i had physical love only to her"; i don't even know why i did that; quarantine was stressful and my girl wasn't warm enough, not cuz she is not interested, but mostly kinda lacking experience.

I'm so doubtful, she knows about how i had a chat with this girl (and the fact that she invited me for a dinner after quarantine, that i changed it in a dinner with a group of people), and what happened yesterday: i mean the fact that the ex boyfriend is angry with me for the screenshot. Of course i hid worst things, but i know she is frail about these things, but i feel so dirty about it.
Even tho' many time happens to me that a girl leave me for another one even after much time of "relationship", but sure i didn't want to have revenge...

I always blamed who cheated, but i'm not different.