TLDR: Girlfriend cheated less than a month ago with a guy she met online, been profusely apologetic. I'm completely lost and have no idea if I should give her one last chance or end it.

Girlfriend (22F) of 1 year confessed to me (29M) last night that she cheated on me less than a month ago, with some guy she met on social networks.
I've been an empty shell ever since.

For some background, she likes to meet people online and talk through the apps. I'm not the jealous type nor a control freak, she's a free woman.
She also regularly keeps me updated on her new friends, without my asking, as if to let me know she's not doing anything suspicious (which could be suspicious in itself, I'll give you that).
I've never been worried, and she even instantly blocks the guys that don't respect her boundaries or when I have the feeling something's off. I'm a dude, I know what dudes want.

Anyway, I was doing the dishes last night, when she comes out of the bathroom not her usual self. She seemed worried, and I asked her what's up.
She tells me to sit down and that she must talk to me about something important. I instantly thought she had done a pregnancy test and that the result was positive, since we had a pregnancy scare a few months ago.

"Remember Thomas?" (Not the real name)
"Sure" I answer.

Thomas is a guy her age she met on Instagram, and they do the same job. They regularly exchange ways of doing things, and he honestly seemed like a good guy.

"Well, he's been harassing me and threatening to look you up on Facebook to tell you we've done things together."
"And?"
"And I'm scared of what he might say, and your reaction."

I'm not the type to get angry easily. I'm tall and strong, and most people don't take their chances at even trying to piss me off.

"We kissed."

Time kinda stopped around me for a moment. I couldn't believe it.

"I didn't want to, it was a moment of weakness."
"Is it all that's happened?"

She doesn't answer and her phone keeps ringing from the message notifications from Thomas. I feel something's really off.

"Is it all that's happened?" I insist.

She gets up because the question clearly made her uncomfortable.

"No."
"Tell me."

She pauses.

"There was penetration."

At that instant, my world collapses.
I feel like Ali and Tyson teamed up to beat me up.
I feel numb, disgusted, empty, angry, powerless...
I haven't eaten nor slept since. Its eating me up inside, my head is a mess, and I don't know what to do.

I love her, we have projects together. We planned so much stuff. Her parents love me, mine love her, and they get along so well. She loves my dogs, and they love her back.

I believe her when she says she rejected him almost immediately and felt disgusted at herself.
She's been nothing but apologetic ever since, and I feel like she's sincere about it.
But something broke inside of me. All the trust I put in her was smashed, and I have no idea whether I can glue the pieces back together.
Perhaps time is the answer. But, right now, I can't even look her in the eyes, even less so hold her in my arms. I'm totally apathetic.

So I'm turning to Reddit, I need some advice, maybe stories about people who gave their SO one last chance and how it turned out.
She's away at her parents' for a few days. She gave me the keys to her apartment, and I don't know yet if I should go there and gather my stuff, or give her that last chance.
She says she'll do anything to keep me and be with me, that she fucked up and isn't looking for excuses. I really want to believe her, but I don't know if I can do it, if anything can be salvaged from the relationship.

She's the best person I've ever known, the light of my days. She got me to quit smoking pot, helped me out of a major depression by loving and caring for me. I love her to the end of the ever expanding universe, which makes the decision so difficult.