Wow, that is just like what it is like for me too. I have to pre-screen everything before it comes out of my mouth, and the higher the level of scrutiny, the less I can let out. I've been having a lot of trouble with talking in therapy, too, and several weeks ago noticed that it felt the same as when I tried to talk to my parents and never managed to talk to them.
It is very different to talk about someone else. I can talk about other people, or to other people about their feelings and thoughts and everyday stuff. The problem is when it comes to talking about me. I, too, can connect with some people easily, with empathy and they seem to feel like they can open up to me. Not with everyone, but some. And not with large groups of people - I'm much more comfortable in small groups or one on one. I'm an introvert, and I'm highly sensitive. Are you? I wonder if that's part of it too.
It is very frustrating. Writing things down helped me a lot, so that's worth a try. Email is much easier too, but I use electronic communication as a crutch. Crutches are fine when you really need them, but not when they keep you from strengthening a part of you that needs to be stronger. So, a caution there.