I know what you mean CLKD about people accepting when you cannot always stick to your commitments! I'd an acquaintance from uni who'd arranged a meet up - this was about 10 yrs ago - can't remember what the event was but my dog had died at almost 14yrs and we were all in a state. I baled out at last minute - as my girls were inconsolable. She was really angry and let me know it. I was quite ruthless but wouldnt do a thing differently if it happened again. I sent her a card and thanked her for her friendship but ended it right there. No compassion and a stickler for rules.
When you are living with something as unpredictable as poor mental health - you need the flexibility and understanding of others that allows you room to manoeuvre-
Shame about the self service tills- my goodness I've two local petrol stations that are unmanned now-
drive in - fill up and pay at pump - and drive on
no need to speak to anyone 
We haven't had manned petrol pumps in Australia for years!!! It must have been phased out here at least 20 years ago. I think some supermarkets here are realising that self serve checkouts are leading to major theft issues and some are returning to manned checkouts.
In regard to your comment about baling on an event with your friend. This just seems silly. If you have given her a good reason for not turning up, well that should be enough really. My father never went to anything when we were children. His bi-polar made him extremely anti-social and it caused so much difficulty for our family. Poor mum had to make excuse after excuse when she really should have been honest. Everyone knew dad was a bit strange anyway! My older brothers used to tell their friends that dad could be a bit tricky. One of brother's friends actually made a big effort with dad and years later when dad got on better treatment, this man used to go to mum and dad's house whenever they needed any handyman work done. Dad used to sit and drink tea with him and I am so thankful that this man made the effort with dad when so many others just wrote him off.
Loneliness does funny things to people. Dad passed away last year from cancer. Mum was a medical receptionist for 20 years and despite this, avoided doctors as much as possible. She is 88 year old and only takes 1 tablet a day for blood pressure. Anyway, after dad passed, mum kept getting little health issues. I rang her local doctor and spoke to the practice nurse. The nurse asked me whether I though mum was grieving rather than actually ill. Turns out that despite living with such a difficult man all her life, mum was lonely and missing him. Mum has more friends than anyone I know, She is a committee member of two large community groups but I was noticing that at night she would call me. We used to talk on the phone a few times a week, but mum was making any excuse to call. I spoke to my older brothers and we all made an extra effort to get round to see her. I live 15km away, so I am the closest, but my brothers rang her more frequently. I as a bit insensitive. I assumed that dad's passing would lift a huge burden off her shoulders, when in fact, she told me she didn't mind taking dad to his appointments, etc.. as they gave her purpose. Oh and recently mum was made Senior Citizen of The Year in our local community for all her years of volunteer work. Now she has to attend functions associated with this and has another purpose.
It's far to easy to write people off. We endured 40+ years of dad's bizarre behaviour, only to find that modern medication made him pretty close to normal. In the few years prior to his death, dad was on Epilim which controlled both his bi-polar and his migraines. He did take a couple of other drugs as well, but he became pleasant to talk to and deal with. I should add that my father held one of the highest public service positions in Australia whilst he was so sick. I still don't know how he did it!