The disconnect between the language you use ("a poor conversationalist" is a very neat and sharp way to describe the problem, and the vocabulary is advanced) and the structure of your sentences and paragraphs and your subjective self-assessment is ABSOLUTELY STARTLING.

I do not know how long it took you to write the post. Maybe you took your time. So I cannot say from one post whether it is reasonable to believe that you are a slow thinker. You do not immediately strike me as someone who is a "slow thinker", but perhaps this is due to the difference between a written soliloquy and an oral dialogue. Is that right? Do you not know how to respond in an oral dialogue? Well, you do - you said that much...

On a different note, is it at all possible that you overwhelm people with your advanced and expansive vocabulary during oral interactions? While "a poor conversationalist" is sharp and neat in writing, and I thoroughly enjoyed reading your thoughts as expressed in writing, I can see myself being put off and a bit intimidated if somebody would talk to me like this while chatting casually. You need to quickly skim (no need to read in depth) this:

Register (sociolinguistics) - Wikipedia, the free encyclopedia

The part that is especially pertinent is in the beginning: "In linguistics, a register is a variety of a language used for a particular purpose or in a particular social setting."

So it could be that you do not have a natural ability to match the register you use to the social setting appropriately. If that is the fact (it is worth entertaining the possibility of your having this problem as a working hypothesis), you probably need to TALK LESS and LISTEN MORE. Also, use non-verbal expressions - nod to confirm that you are listening, keep an eye contact, etc. Try to notice how other people speak (they are probably much more relaxed than you are, possess the natural ability to match the register to the social setting appropriately, and do not hold themselves to your high standards of needing to excel at being a good conversationalist). Since you do not feel particularly uncomfortable around people (that is lucky), you should be able to ease into conversations by sticking to the role of the listener for now (you do need to make minimal utterances, though, in order to appear normal. "I see..." is one of such minimal utterances that comes to mind).

Being more of a listener than a talker, for now, would relieve some of the pressure on you:

1) "I can't think well under the pressure of people waiting for a response."
2) "In group settings my almost constant focus is on how I am being perceived by others."

So... let people talk. Many people really like to talk. Not everybody, but many people do. These people would not waste their time on "perceiving" you - they would rather have you listen to what they have to say.

Try those practical approaches - you need to start solving the practical problem, regardless of the personality labels.