Print out a big poster, one A4 sheet per letter saying " She does not love me and no matter what I do she never will". Stick it on your bedroom wall and stare at it until it sinks in.
I've been in the same situation mate. All those people that say, go out and get laid then you'll be cured, haven't been in love. There is NO easy answer. My ex left me two years ago for another bloke, I became an alcoholic and depressed etc. I did finally get over it.
My advice...
DO NOT contact her - it seems simple but it's the most difficult thing you will have to do, probably worse than giving up crack.
DO NOT forget how she feels - she does NOT want you or fancy, your brain will keep telling there is still hope, "what if". It sounds harsh but there is no hope. Think logically (it's hard), if she texted to say her sink was broken you would be over there in a flash. You WILL misread this as she still wants you in some way, she DOES NOT.
DO get on with your life - you're in a hole, you have to try and dig yourself out one small step at a time. Try to get in shape, when you're down it's twice as difficult as you have no motivation, but fight it. You say your flat is crap? why? because she's not there? Why can't you do it up and make it look nice, you did it before. This is the wrong time to meet other women, it might even make things worse, but keep an open mind.
DO remember time heals - each day that passes you will get a tiny bit more over her. A day will come when she is out your system.
So one girl doesn't want you, I bet my life that in time there will be another as good if not better. Remember, this isn't your mum talking, it's a anonymous person on the net. I couldn't care less about you, I'm just passing on what I've learnt.