I found it quite helpful to write things down, as I would send it to my T. But then again, I decide to keep it with me for another day or so. But I promise myself that as soon as it's on the paper or in an email, I let go of it (until the next day) and do something nice for myself. Like last time I went for pizza instead of having my appointment.
Then the following day, I go through what I wrote down once again and try to notice whether anything inside of me changed. Are the feelings still that strong or do I think it was a bit exaggerated? In the first case I might still decide to send it to my T after all. In the second case I give it a bit more time and quite often, by day 3 I don't consider sending it anymore.

It can be very helpful not to deny what you feel, but also not to give it a chance to linger. Write it down and let go of it. But, more importantly, give yourself the opportunity to get back to it if you feel like you have to. :-)
Also if you decide to send it... Once it's gone, it's gone. Don't start thinking 'should I have sent it or not?'. Because in most cases the feeling won't stay that strong and then you blame yourself for having sent it maybe (which happened a lot to me). Please don't. Try to see that, at that moment, it felt like the right thing to do. And then when you or your T wants to talk about it, you can. Sometimes I think it's quite useless afterwards, because I'm not this upset anymore. But it's not. You'll realize this by the next time it happens once again. At least, that's how it works for me. :-)