I'm in a similar situation with my MIL...
Over the years we have bent over backwards to accommodate her and that is always conveniently forgotten (think spending all our spare holidays at hers, travelling and staying for whole weeks at a time when she has been ill etc).
Obviously now we have 2 children and a dog, as well as DH hardly having time off, we can't visit as much as we would like. Our only holiday last year was 3 days in a caravan,which we chose near to MIL so we could spend one of our only holiday days with her. This wasn't enough and she still sulks about it now 
But the competition she has (in her mind) with my mum us unbelievable. And my mum has been nothing but kind to her, even as far as inviting her with us one Boxing Day. Anything I mention to do with my side of the family she makes a cats bum face. It used to really upset me, now I realise that moving so far away has made her overthink everything and brood over every little thing.
There really is nothing you can do,DH has tried explaining things until he's blue in the face but she is so used to feeling sorry for herself it doesn't make a blind bit of difference.
We are off for a weeks holiday in Cornwall next week with my mum and family (splitting the cost of a cottage so we can afford to take a holiday for the first time in 4 years). MIL has practically imploded at the injustice of it all 
I actually get on very well with her, yet this trait drives me round the bend! All I can suggest is treat it like the childish tantrum it is and try not to let it get to you. Easier said than done, I know!