Could a rotting tooth cause painful glands in neck off and on for months? sometimes the pain feels like it's around collarbone.

It can: It could be referred pain or a spreading infection. You already know for months that the tooth needs to be treated... Why are you waiting? See a dentist immediately.

Infection: Unless you get the infection cured you run the risk of the infection spreading to other organ systems. You seem to have a virulent infection requiring immediate treatment. The infection will not self-cure, see a Dentist now.