OK so I slept, and the person in question has got back to me and accepted my comments so while I still feel a bit tender and uneasy the sense of panic and doom has worn off.

I have to explore this further though, why do we do this? Rationally I know I had a valid point but the irrational part of me just goes to pieces if I try and stand up for myself when it's for me (let's be clear, I can be very assertive when it comes to the needs of others such as my students or political causes). I know how to do the assertiveness thing, but as soon as I open my mouth about my needs straight away I start to think it must be me that's wrong and go to pieces, and if the person at the other end retaliates I'm devastated and usually completely give in and go beyond what is needed to make up to them.