you get pretty decent relationship advice in the sink; mainly because you get a wide range of opinions and you can pick and choose what to use.
here's my 1.13 inflation adjusted cents.
you're both mature in that you talked it over and explained how you feel instead of yelling, screaming and pulling her hair in your spiderman underoos. this is a pretty positive sign.
it's clear what she wants:
she's not sure if she wants to stay. she's going to try out some new cock to see if she can make a stronger emotional attachment with the new guy.
this reassessment of the relationship appears to be triggered by your "rough patch". things are tough and she wants to bail. that's normal.
that's fine. let her roam - you can't really stop her.
she'll a) do it --with-- you knowing, b) cheat behind your back or c) not cheat, but constantly wonder if she should've broke it off.
having said that you need to know what you want. outside of sex, do you enjoy having her around? do you laugh? can you talk about anything? are you comfortable with her? in other words, do you consider her a good friend. if you want a long term relationship, being a good friend is where it starts. if she is, then tell her you want her to stay, but can see she's questioning things and that you understand she wants to try out the new 12" man cock™. tell her you hate she's looking elsewhere, but you won't stop her because she needs to find her answers and decide for herself.
you should probably trust her. she's telling you all this difficult shit when she could've just lied. she shouldn't be punished for having feelings and being honest about them.
the trick is that you can't control her. you should let it play out and give her room. she may fuck another guy. if she leaves, she leaves, if she comes back, then you have control and can decide if you take her back.
if you break it off before she fucks the guy, it's not a great decision in my opinion. she'll leave without having done anything. i don't believe you should punish people for having bad ideas, otherwise we'd all be criminals.
edit: one other thing, you should figure out why she's so unhappy and see if you're --both-- willing to make changes for the better. cheating is a symptom of her being unsatisfied in the relationship. figure out why.