As several have said, go easy with yourself. It sounds like things got bad for you and you did the best you could. I am pretty sure it is not the first time your t has been on the receiving end of an upset client's email. And it does stink that the slot got filled so quickly. I know I have been on the "call list", which means that I get a call if there is a cancellation, but that the t's schedule is too full to accommodate me unless there is a cancellation. I have been grateful to get in when I have been on the call list: it's happened maybe 6 or so times. But each time I have really wanted to get in. Maybe that's what happened here.

I do think that if a client cancels the t is perfectly entitled to fill the slot. I think any professional would do that, and I can see that if there is someone who wants to get in, it is only fair to offer that spot.

So yeah. Be kind to yourself. There a bunches of less healthy things to do when emotions are boiling.