Ugh, I know how it feels to be in this boat and it sucks! I myself have had a big problem with this and still do to an extent. As others have said, it's just part of coming out to yourself, once you can do that it does get easier.
I don't know if it would help you, but I found that I could easily express my thoughts in a letter or an email, just not verbally; I tried to make this clear to my therapists by pretty much saying "Hey, I've got something I want to talk about, need to talk about... but I can't talk about it.... can I write you a letter?" they always have been ok with it. In the begining I'd say that most of the progress I made was thru this method; I dunno... for me, once the thoughts are out there I'm forced to deal with them and things aren't that bad, but esp. in the begining, it took someone else starting the conversation for me to be able to open up at all.
As I said though, it does get easier. It still takes me a while to get started in a session, but once I get going I just blabber on and on about everything, sometimes I just zone out talking and don't even know what's coming out of my mouth. I'm sure I'm annoying in those sessions, but I find it therapeutic!
~Sara