I (36M) caught my girlfriend (34F) hiding a small pack of condoms on her backpack. 14 years together

I (36M) caught my girlfriend (34F) hiding a small pack of condoms on her backpack, this saturday 17th june

Of course, after that I confronted her and asked for an explanation. She initially denied it but after pushing her, she admitted this: there is a coworker (married w/kids) that she is attracted to. Things evolved, they started sexting, and a week and a half ago they made an attempt to fuck. She says nothing happened, she was on her period and also she felt bad about it at the last minute. This was on friday 9th according to her story.

One week later (17th june) was the day I found the condoms by pure chance. And I know (ticket was also there) she bought them that very saturday.

I’m looking for advice here, or any kind of input. We have been together for 14 years now. I never considered even possible for her to do this.

Is it even believable that they did not do anything? Even if they didn’t and I believe her version, how can you explain buying condoms after the failed attempt?

She is asking me to forgive this, claiming that absolutely nothing happened.

PS: BIG THANKS you all for the incredible support. It is honestly an enormous support for me at this time. Of course, every story has its backgroung ans specific details which I chose not to share, but the comments have been a top-tier help for me to gain mind clarity. I have good reasons to believe her story, even considering our DB periods, and believe this was only this time, but still I cannot see any scenario other than a clean break. I also have good reasons to think she needs mental help, which I cannot burden myself with.

Again, you have been a great help, I cannot state this enough.