I used to help coach college friends in their writing. Often their first drafts would have complicated pretentious sentences that really weren't even parseable as sentences, and first I'd ask them, "What do you mean here?" And then when they'd explain, "Write that down!" Perhaps something like that would work for you, to help take you from a "composing" mindset to a "conversing" mindset. You could even record yourself or use some dictation software. It may even make your writing easier to read.
It might also reassure you that professional writers often say, "There is no writing, only re-writing." The first draft is never the final draft, so you don't have to take it seriously. Just jot down whatever comes into your head. Some people start with an outline, scattered words with arrows connecting them, questions to answer, blank spots, etc., whatever helps to keep you moving forward. Getting started is the hardest part. Keats used to chain himself to his desk to force himself to write something. Sometimes it even helps to set yourself a silly challenge, like randomly open the dictionary ten times to pick ten words you have to include. They don't have to make it into the final draft---but they could. :-)
You might also want to read some books about good writing. Strunk & White is good. Their advice is "keep it simple." Clear and Simple as the Truth is sort of a step past that to a slightly more artful style. Another book I enjoyed was The Artist's Way, which despite the title has a lot about writing.
I don't know anything about professional therapy, and perhaps these suggestions are all way off the mark for you, but I offer them just in case you find them useful. I'm sorry that writing is so painful for you!