In light of how many doctors and tests you've had that have all come back negative, I fear your anxiety has taken a very strong hold on you. I can definitely relate to what you are going through because I've also developed obsession with various lumps, bumps, aches and pains.
The most accurate and revealing test you had was the CT with contrast. If ANYTHING had been wrong in that area, that test would have picked it up. I know how easy this is to say, but I think you need to accept what the doctors have told you about this particular lymph node.......that it's just "odd" but is of absolutely no danger to you nor is it indicating that something is terribly wrong. I'm sure one of the doctors you saw told you to stop poking at it........but I also know how hard THAT is.
In your list of doctors, I did not see that you had consulted with an immunologist. These doctors are specialists in the lymphatic system and if anyone could tell you what is going on with this one node, it would him/her. But as with any specialist, they are not inexpensive to see, but if this might possibly give you the peace of mind you finally need to let this go, it may be worth it.
You asked if this was a "mental thing" from thinking about it and I believe your question also asked if anyone knew how to stop it.
Have you ever considered getting into some therapy for your anxiety? Like me, yours also seems to include some hypochondria. Those two combined can really make life miserable. I hope you will consider it.
Just know that IF anything had looked bad about this node, the doctors would have done something.
Keep in touch and I wish you the best
RubyWitch