Hi arbbarb,
Thanks for the support.

What actually makes it worse - and this probably isn't a detail which I should have omitted - is that she actually DID give me a chance, multiple times. She sat me down, told me that my way of complaining can't go on, and that I need to change something. I listened to her, and for a couple of days I vowed to make it all better; but soon I was back to normal.
I think it needed a drastic cut like this to wake up, no more comfort to hide back into.

She actually never told me that I was "her world" - it's always been the other way around, exclusively. To make matters worse, she had kind of an "emotional affair" with another guy as well. There was never anything physical between the two, but they were flirting. When she broke up I asked her if she had feelings for him and she claimed "maybe a little".
The thought that she is now with him, probably happy, is just so unbearable.