I know you are very scared right now, I felt that way with HA many times. The fact is, I am not a pep talk guy, more of a in your face guy, so take the message the right way, it is meant to help you.
How in the world did you connect these symptoms with brain tumor ?! Nightmares? And headaches? They are ridiculously common, brain tumor is not. Neck pain / shoulder pain, point out to some tension in the muscles, probably due to sleeping the wrong way because of the nightmares. Guess what, headaches come from that too. That is logical thinking. You've got the medical opinion as well. So, the only thing you've left with is the feeling you have something serious, which doesn't mean s***, because it's a feeling caused by HA.
What you should do is work on your anxiety. My first advice, stop googling symptoms completely. Second, if you can, find some good therapist and visit him.