Off subject but good for you.

Cheating is always bad. If you get into a relationship with someone with the understanding that you'd be exclusive to each other, then there is no excuse for the dishonesty of cheating. If you want to end the relationship or pursue other options, be honest and end it. Cheating to me is just a cowardly way out or just a complete betrayal and breakdown of trust.

Your edit was after my first post.

I'm gonna be foreal with you though, the fact that you've kept something like that from your girlfriend for months is already pretty bad in itself (how would you feel if she did the same thing and hid it from you for months), but it also seems like you're only worried about protecting yourself rather than telling the truth, since you seemed fine with hiding what happened until now and are only worried that she might find out from someone else. You even said that you don't want her to find out from anyone but you, but it's been months already of you hiding it, seems like you never even had plans to tell her in the first place. 

You should have just told her months ago when it happened. You've made the situation a million times worse now because of that, now she might think you've been hiding other stuff from her too. It's up to you on what you decide to do though, just saying my thoughts on the matter since you decided to share the situation.

There is no excuses to cheating. To me if you cheat on someone and risk your relationship for sex with someone else than you don't like this person enough to spend your life with them anyway. So if you feel like cheating just end the relationship.

Wow, people are so extreme here. I’m surprised.

As for me, I’d certainly forgive. I don’t understand why two people should go through hell and suffer/be tormented over a mistake. I think it’s really childish to end a relationship, especially solid ones, because of one or two mistakes. You know, we’re not perfect and the temptation is there. I never cheated, but I can’t say for sure that I never will.

Has it ever happened to you, though?

No, btw I updated my post to give some more insight.

I want to protect the relationship, sure I want to protect myself from the hysteria and fights that will come out of it too but I don't want to waste so much energy on something I didn't mean or even set out to do. She wouldn't understand because she is a bit conservative and possessive when it comes to relationships and will no doubt take it to bombastic proportions. I've just got to survive until the weekend and during the weekend I'll most definitely stay close to her while we're all out. 

I know that if she finds out from some other person, I'm probably going to have to pack up and leave for the countryside haha.

If I tell her, it's just going to be ugly. Either way, it'll be ugly actually. 

I kinda don't want her to find out ever, even if we break up, that might be selfish but it'll only bring pain and loss and nothing else. 

Maybe it will be ugly but don't you think she deserves to know at least though? Again, if the situation were reversed and she had done something by accident, wouldn't you have wanted her to tell you? Like you said, it will be worse if she ends up finding out from someone else. Either way though it's up to you and what you do, if you don't ever want to tell her that's your call, I just think honesty is the better option for a situation like that. 

I have to say, it'll be much uglier if she finds from someone else than if she does to you. I guess you could never tell her,  but if you're someone with a guilty conscience the sooner you tell her the better as the healing process can start earlier.