Dude. I don't want to be evil, I really don't. But this whole thing is a major dealbreaker in my eyes. Let's go through the points based on what I think:
2. Lately our relationship has not been going well. She said that she is not happy in the relationship, has been depressed and we were on the verge of breakup. But we decided we should try and make it work because we love each other.
Being "not happy" and depressed is not an excuse for cheating. You get the fuck up there and tells what is wrong. If it doesn't work, you freaking break up. Cheating is never excusable.
Plus, I read this line but I understand "I love her a lot, but she doesn't love me as much, even though I think she does."
3. She said that she had sex with a friend of hers, not too close. It was a one-time "casual sex" during a holiday she took.
This hurts just from reading. Like Zangetsu put, this reeks of premeditation, even if there was none.
Do you even know who this is? Because if you don't, she SURE AS FUCK won't be kicking this dude away to never see him again. He'll be always looming near, and will even get to see your face someday. And you'll never know who he is so you can
at least avoid his presence and preserve yourself.
4. She was drunk. (not an excuse of course, we all know we control our wishes while drunk; it is only judgment that becomes clouded).
Even before drinking, everyone has the power to choose to have their mind clouded or not by booze. This, by itself, is already enough for me to end everything under any circunstances.
"Oh I'm sad. What do I do? Maybe get drunk so I lose control of my faculties? Yeah, sounds like a good idea to me."
She says she regrets it, never did it before and will never do it again, and wouldn't have done it if she know how bad I'd feel about it.
Been there, heard that. Didn't work with me and I was right all along. Sure, everyone is different, but the "once a cheater, always a cheater" cliché exists for a reason.
And she regrets it. So she IS aware that the casual-sex thing was a negative thing to do, but still did it? One more reason this is a major deal breaker with me - trying to pin the blame on you. Like you said in your bottom line, you had NOTHING to do with this. You're the one being cheated on.
C'mon man...
5. She said my position on casual sex was unclear. Partly true, because while I don't personally like the idea of casual sex, I said it's not a dealbreaker. But that was probably because I never seriously thought she could do it; When she said she had, I felt terribly disturbed and disgusted by the thought of her having sex with someone else.
This is a complicated spot.
Learn from this: If ever, EVER,
EVER your position about casual-sex-while-being-in-a-relationship is called into question, and you feel that you are unclear on the position you'll take, the answer is NEGATIVE. Say you loathe it, even if you really don't.
I also find this whole "you were unclear, therefore it was green flag" instance extremelly dishonest. Generally, the default position for cheating is NEGATIVE. But beyond that, she goes around doing crap and tells you LATER to see how you'll feel? WTF MAN?! She goes around filling voids in her heart with casual sex? Awesome girl!
I think you should part ways, because you're trying to resurrect a dead, putrid dog. But bear in mind this all is just MY vision of it based on what you brought us.
And I should say this: Do not buy the "I won't do this again" spiel. It's not about doing it again, it's about doing it ONCE. And even if you forgive her, by my experience, the next time she does it, she'll pin the blame on you even harder.