a brain tumor would cause symptoms much more obvious than just a headache, and brain tumors also don't come and go as they please; someone on this forum described them once as "persistent and bossy" and that they MAKE themselves known through symptoms that i won't go into detail about so as to not trigger you. but trust me, dull headaches are NOT one of those symptoms.
a vast majority of headaches are completely benign and caused by stress, sinus issues, lack of sleep, poor posture, stiff neck muscles, and a multitude of other normal things we all go through from time to time. chronic stress is one of the biggest contributors to tension headaches which can easily occur across your forehead as you described.
as for your blood work, i'm quite sure that any sort of cancer or malignant mass would alter your blood results and cause your doctor some alarm and to order more tests. i know it's frustrating that he won't give you another head scan, but think of it as a good sign that he isn't concerned about you having a tumor or being seriously ill. doctors tend to know more about the health issues we fear than we do, after all.
(it's ironic that i can reply to this thread so calmly when brain tumors and cancers are my current biggest fear as well. i can definitely relate to your anxiety when it comes to this.)