I've n/c for this as too many real life details but they are very relevant. I'll try not to dripfeed but this could get long...
DM is becoming increasingly hard to deal with and I don't know what to do. She is kind, generous, proud of me and would do anything to help me. The flipside of that is that she is needy, demanding, attention seeking, very easily offended, tries to organise the lives of those around her to an uncomfortable degree, falls out with people with depressing regularity and can generally be quite difficult.
The background to this is that she had a difficult childhood (hints at abuse but I don't know the details) and an unhappy marriage to my father followed by a very abusive second marriage. She also lost a child - my sister who was killed in an accident aged 8.
I know someone can't be expected to be 'normal' after all of that and I try to take it into account as much as I can because I think it does explain a lot about how she is, but there are two things that I think and I just want to air them here as I can't do it in real life:
1 - How much can an unhappy past be expected to excuse unreasonable behaviour? Do you have to keep giving the person the benefit of the doubt because of what's happened to them or does there come a time when the past can't be blamed for the present?
2 - (and this one really gets me) ALL OF THOSE THINGS HAPPENED TO ME TOO! (Apart from the suspected abuse - my childhood was very happy until my sister died and my parents divorced). But I lost a sister and I lived in fear for the whole of my teenage years with an unpredictable and terrifying stepfather, a situation that I had no choice about. I sometimes wonder how life would have been different for me if those things hadn't happened - Would I have been more successful? Married someone different? And then I realise I am doing the same thing - blaming what's happened in my past for how things are now).
My maternal grandmother recently passed away which I am very sad about but it has also opened a can of worms in my head about my DM's relationship with her mother, my relationship with mine and also my relationship with my own daughter.
I don't want to repeat the mistakes of the past but I don't know why I feel so unable to pull my DM up on her (sometimes) unreasonable behaviour. I'm like this with other people sometimes as well - taking crap from friends rather than saying what I think - and I don't know why because I'm generally quite a confident person and have no trouble bollocking my poor DH on a regular basis. 
I think part of it is the fact that DM claims I am the only person who's always been there for her 100% (little does she know) and that is such a weight on my shoulders - if the only person who has always supported you 'lets you down' - and I could do that in her eyes by making just the smallest 'mistake', something nobody else would bat an eyelid at - then what are you left with?
I am so scared of messing up my relationship with my DD as she gets older - it would be awful if she resented me the way I do my DM. I also know that the time will come when I have to care for my DM as she did hers and I'd like to do so with good grace and not be secretly fuming at her while maintaining a martyr-like outward appearance of a dutiful daughter (as DM did with hers).
I don't want this to affect my family - DH finds her very hard to deal with and although he has his faults too I can sympathise to some extent. Talking to her is not an option so I need to find a way of dealing with it in my own head so that doesn't happen.