I'd bet you all the money in my bank account she's cheating.

So...now what?

Here's what reddit will tell you: move out, get an apartment, bail immediately, just run. They aren't wrong.

But there are other options. One of which is being willing to hear her out if she's honest. Let her tell her truth to you. Yes, she fucked this other guy. You know it, she knows it, let's not play games. You know, you want to hear her out. So...hear her out. Hearing her out doesn't preclude leaving her (which, in total honesty, you'll probably do) but you can at least talk like adults.

What went wrong? When did she feel like this? Why? Get some honest answers. Get a feel for what happened. Don't fall for the "I've always loved you/I'll always love you" bullshit because that's dead now. But seriously, try to talk about it. If she can't do that, fine, then resort to reddit option one which is running far and hard. But I always vote for TRYING to talk first, at the very least, provided you can stand to hear.

Maybe you'll learn some shit that's useful.

If you're being cheated on and dumped, you might as well walk away with a learning experience. Or, a chance at healing if such might be available. Most infidelity ends relationships. Most relationships have infidelity. It is just that the ones that survive go unnoticed. So....you have some choices. Run, talk than run, talk then work, ignore. You have choices, and those choices are yours. They are totally up to you. But six years is a long fucking time. I'd want to know. I'd want to find something to understand.

And if you want that, then you have to be kind. Because you won't get any answers if she's defending herself.