How have you been sleeping? If you're anything like me, the stress and worry is causing poor sleep, thus leading to the sluggish feeling throughout the day.
I have sinus problems myself and I wake up many times groggy, with a dull headache and an overall feeling like I have the flu. It normally fades in a few hours as my sinuses drain. I'm not sure if you are using any type of nasal sprays for your sinuses. I have not had much success with liquid sprays which drained down the back of my throat and made me nauseated. I like Qnasl which is spray powder. I also really like a simple saline nasal spray in the morning that clears out the stuffiness and helps my sinuses drain.
I am fairly new to sinus problems and when they first developed it was scary. I too jumped to the worst conclusion naturally, only with me it wasn't a brain tumor but naegleria fowleri (aka brain-eating amoebas). Of course I KNEW I had all the symptoms of an amoebic infection, the headaches, the stiff neck, the nausea....
Sinus issues, like I said have made me feeling very slugglish and run down. If you doc isn't sending you for a head scan, your symptoms are not raising a concern. I know its hard to accept this and to trust someone else. Believe me, I know because I also do it all the time. Just try to trust your doctor's expertise and do not google any symptoms any more. There is an excellent pinned thread in this forum about googling symptoms that I have been reading when I feel the urge to hit up good old WebMD.
A dull headache across your forehead is a common but harmless symptom of sinus problems. Have you been to an ENT? They may be able to help you out with your sinus troubles better than a GP.
Hope you feel better!