"After dinner we went into an arcade. We were walking around and talking and I said something stupid (as per usual) so she slapped my arse, I slapped hers back laughing and she turns around. She tells me that we can't do this thing anymore. The way she said..."

Go have anonymous sex with strangers and go on a big bender. You'll forget about her, and kill your liver so it's all good. But that's a bummer dude. 

What's so bad about this? You avoided marriage. Your life will be kept in safe status.

Argh, I thought I had it bad.
 
I feel for you man, I really do. Casual relationships almost never seem to work out from what my friends tell me. You both have to be on the exact same page, all the time. At that point, it might as well be a "serious" relationship.
 
My advice is to go grab some of your buddies and just keep yourself busy for the next while. Have fun, go drink, do some dumb shit that you may or may not regret the next morning. Play Demon's Souls drunk like I did a few weeks back. Just keep your mind off things for the meantime. This girl seems to really, really not be right for you (I mean getting engaged after barely knowing the guy? WTF?), so its best to just keep going your separate ways for now.

" Cool story br....No. For once I'm not going to be a snarky dick.  Sorry man that sucks, loving someone who doesn't love you back is a unique kind of pain. "

i feel for you brother

" @Synthballs said:

" @TheMustacheHero said:

" Dude that's great news! Congratulations! "

For some weird reason this made me laugh. Thankyou. "

You're welcome, son. Just know this.... There are people somewhere that are so ugly they will never even make contact with the other sex.   Also fish in the sea. "

the fact that she's saying this after only a week (or less) makes it seem like she's not worth it. I know it doesn't feel like it now, but eventually you'll be able to look back at it and realize what a mistake continuing a relationship with her would be. 
Also, the blue eyes comment when she knows you'll take it the wrong way is just a bitch move.

" I sometimes have to remind myself this is a video game site. So many kids/teens writing about there world ending girl troubles. :P  Perhaps that makes me sound like a dick but damn. It's not the end of the world mate, get over it. Suck it up.  edit: Andd i just realized this is in off topic so i suppose this being a video game site means zilch. Still! Point remains the world will keep spinning. ;D "

Sorry to hear, most of us have our fair share of heartbreak, your not alone.

I know how you feel. I have this one girl...

" @LiquidPrince:  And...? "

After dinner we went into an arcade. We were walking around and talking and I said something stupid (as per usual) so she slapped my arse, I slapped hers back laughing and she turns around. She tells me that we can't do this thing anymore."

" @VinceNotVance said:

" @LiquidPrince:  And...? "

Complicated situation, but my heart literally hurts when I think about what's happened. Never thought it was possible to feel a heavy heart. "

i feel you too, only the best to you in your life!

Thats shitty man im sorry.
 
But its apart of life, you will come out better for the experience in time.
 
Just be wary of her coming back into your life as it suites her, that will fuck you up over the course of years if you dont make a definitve stand.