"I have a terrible time finding the right word. I'm writing along and I want to say something, and I can't think of the damn word! It makes writing so slow for me that I can barely stand it."
This anguished comment comes from, of all people, my own father, who is actually quite a capable and insightful writer. He's worrying about the right thing--but at the wrong time.
When you're writing first draft copy and you can't think of the right word, that's your internal editor talking. You cannot afford to listen to your editor when you are writing first drafts. His time comes later, when you're making revisions.
"But I'm looking for a word! How can I shut that off?"
You shut it off by not looking for that word. Use another word, or just put an asterisk there--and keep writing. That specific word just doesn't matter. What matters are the broader thoughts you are putting down. Keep moving.
Remember, never listen to your internal editor. It will crush the life out of you.
One final thought, which is a truism for anyone who has edited their own work: We know during the editing process that we'll change a ridiculous number of words and rephrase a ridiculous number of sentences. We will make significant changes that never even ocurred to us while we were writing our first draft.
Think about this for a minute. Consider how arrogant it is to think that your internal editor could possibly know what "the right word" can be when you draft anything for the first time. Right now, at this moment, you have no idea what future changes you will make to that sentence.
Get over yourself and keep writing.