When you have good reason for feeling angry towards someone who has messed you around and knows better, how can you let them know this in a way that doesn't damage the relationship?

It's okay to let someone know that you are hurt and angry. For one it lets them know that you care about the relationship. It's not such a good idea to pretend that you don't care if you do. And talking helps sort out most misunderstanding. Just let him know honestly how you feel about what happened and about how you feel about your relationship. If he is into the relationship as much as you are he will understand.

that's a beautiful avatar and yes, you're absolutely right.

Sometimes I feel reluctant to state too many of my needs out of fear of spoiling a relationship by seeming to be too demanding, and so possibly losing the trust and affection of the person I love. I can also easily jump to conclusions based on assumptions I've made to do with what has been said or jointly expected, and not first ask some simple and straight forward questions. I often find I've made a mistake about certain assumptions (such as yesterday), so it's important for me to first wait and see what the situation is about before losing my temper (which thankfully I haven't yet done in this relationship).

I'm finding I have very different expectations to my boyfriend over how plans for spending time together are made and how we communicate those plans ~ I'm wanting both of us to come up with ideas about we can do in our free time and to talk about it, to then be clear on arrangements and getting the timing of messages to-and-fro right so that neither one is left wondering. I can't bear to be left hanging until the last minute, waiting on him to decide what he wants to do. By the time he lets me know what he wants, my imagination has run the full gamut and I get angered at what seems to me his indifference towards the value of my time.

I think I'm starting to see more clearly how he and I differ, and I'm hoping we can make a few simple adjustments to save us both from going through unnecessary emotional conflict.

Thanks again. I appreciated that.