Meril,
I agree, posting on WP will give you feedback, and the opportunity to hear how others interpret your words so that you can practicing clarifying as necessary. That said, your writing is more articulate than many of the emails I get from my college students! Befriend spell / grammar check and it gets even easier.
I also struggle with both writing and speaking. Even a simple email can take 15-30 minutes to compose. However, I do prefer writing, because I don't have the added pressure of another person standing there, waiting impatiently for the next word, or for me to get to my point. When I write I can have someone else do the editing if necessary. I have a very difficult time with word retrieval in speaking, but in writing I can leave ellipses in the middle of a sentence until I think of the right word. Or, I can do this: [I can't think of the right word but it starts with an S and reminds me of ...]. When I think of the word, I can put it in place of the ellipses or the description in brackets.
In a writing class I learned a technique called mapping or diagramming, which works very well for me because it is a visual technique and I am a visual thinker. You start with one word or phrase, which is usually the topic; for example, "Music." (It is usually more specific than that) You draw a circle around that, and then draw lines projecting outward like rays of a sun. Then you write other words at the end of each ray, which are the things you think of which you would like to say about music. You circle each of those, and do the same as before with each one, so that at each level you are expanding and refining each thought and connecting them to concepts which are related. It takes time, but it really helps me to figure out what I really want to write about a topic and what words I want to use to describe it. It also helps you see what is most important, because the most important points are the ones which are connected to more ideas, so you can cut out the items which are tangential. I use this frequently, and won awards for my writing at school. But the quality of the result is, as you said, deceptive, because the effort it takes to get there can be so frustrating, especially when it takes so long (I am up to 30 minutes now with this post!). Find a classmate or teacher whom you trust to be objective, who can give you feedback on your writing assignments.
For speaking, some smartphones now have a voice memo option, or you can get an inexpensive dictaphone / voice recorder. Record yourself speaking about something in advance, and then play it back to yourself. Find something to improve, and then try again. This is especially helpful for practicing conversation; you can sit with a friend and talk together, and then replay the conversation and they can point out moments where you digressed or seemed stuck. There is probably a pattern to your conversational "stumbling blocks," and listening to them with others can help you to identify what is disrupting your course of thought or what concepts you need to learn to communicate. I have a difficult time expressing emotions, so when something happens that I feel strongly about, I work it over many times this way even in my own mind before I try to communicate my feelings to others. Otherwise, it's a disaster...