I find that talking about these feelings can really take the power out of them, so your instinct to talk about them with your T is a good one. I might start by just saying what you said here, that you have these thoughts that you aren't meant to be here and that you fail at things and you're worried about not being taken seriously. I hope your T is really able to listen and talk about these feelings because it can be such a hopeless, difficult place to be in.