I think that you are spot on with everything here. Even though we know something, thinking something else can cause overwhelming feelings.
I tend to just deal with the facts today, what do I know for sure? Well, I know that you have some type of lump on your thyroid that is going to have extensive testing this week. That’s all I know, those are the facts. What other facts are there? Is it painful? How can you ease the pain? Because it’s on your thyroid is it making you tired? How can you practice self-care? So instead of forecasting what may or may not happen in the future about an unknown illness, actually tending to your present-day body will help you feel in control and calm your mind. “Just the facts” are a big tool for me in recovery and then trying not to attach any emotion to the facts either.
I am not sure if you meditate but meditation has saved my butt so much in recovery too. I use the insight timer app and a bunch of us use this thread to check in. You can join in if you like 