I have literally the exact same symptoms as you. I have been sick for what feels like a year. Post-nasal drip, sinus pressure, coughing up phlegm. This lymph node on the left side of my neck has been swollen, and sore, for the entirety of this time. My neck muscles usually feel a bit stiff there as well. I also have pain and a sort of pressure in my left clavicle/shoulder region. The doctor seems to think I'm fine based on blood tests. We're probably both okay, but I understand how you feel, and I too fear that I have lymphoma, a mass in my chest, or something else causing all of this. That something else might be anxiety though. I've felt this way previously, two years ago, when I first encountered anxiety. I thought similar thoughts and had similar symptoms, but I alleviated my anxiety for a while and both went away. It's come back for a brief stay, and I'm noticing the symptoms and thoughts return. A definite correlation there.
I think what we both need to understand is that lymphoma, and other malignancies, present themselves in certain ways. They are rare to begin with, and when they present, a doctor will typically know. It is very rare that something as serious as lymphoma or other cancers will go completely unnoticed for such a length of time. It can happen, but I think typically those cases do not involve a hypochondriac getting a checkup with blood work every six months. Just ask your mom how much blood work can really tell doctors. (Hint: It's a lot).
Again, I understand how you feel, completely. I'm one hundred percent with you. But we're both going to be okay. Trust others. Trust your body to let you know if something really bad is happening. It doesn't want to die just as much as your mind doesn't.
Positive thoughts. :)