I had arranged an appointment with her, but after waiting for two hours at the bar I realised that she had stood me up – this was the first time this had happened to me. I had arranged an appointment with her, but after waiting for two hours at the bar I realised that she had stood me up – this was the first time this had happened to me.

I'd probably use the past perfect in this case, because these are events that happened before the moment you realized (past simple) what was going on. Same as the classic "I had already eaten when you arrived."

It is sad that even people you consider friends may turn out to be crankyunreliable. It is sad that even people you consider friends may turn out to be unreliable.

I'm just not sure "cranky" is the best fit here. Cranky is like when you're tired or hungry and get mad at people easily. https://www.merriam-webster.com/dictionary/cranky https://www.merriam-webster.com/dictionary/unreliable