Firstly i'm very sorry to hear of your situation tom, i am not in a similar one, so my advice is probably not much use but hey? a problem shared, i know there are far more 'qualified' people on here that seem to know all the right things to say in situations like this, me, i'm a bit of a joker, but i have been through situations like this, just not at this moment. However, 11 years is a long time to 'suddenly' want out, do you think its been brewing for a while but you have chosen not to read the signs (its not a criticism) i take it you dont live together, and if there is no-one else on the scene would it harm you to just give her that bit of freedom? Maybe you were her first serious boyfriend and she feels that she has missed out and life is passing her by? God knows i feel that at least 3 times a week, but everyone handles it differently, the chances are if you go away with your mates, (maybe take another girl out - doesnt have to be serious) she may be one of those who doesnt know what she's got till its gone?
Losing your sister mustve been tough, maybe she couldnt cope with your grief? some people cant.
Dont get bitter and twisted about who stuck by who though cos its not a competition, and feelings of anger turn into bitterness, and the whole thing will blow up out of all preportion.
Take a step back ok? bad time of year but you'll get over, focus on yourself for a while, and do your greiving properly.
Sorry if its not exactly what you want to hear, but i really do think you will be fine, it may be that time away from your g/f makes you realise that going out having a good time is just what YOU want too!! ; ) good luck and all the best for xmas and new year