When I had brain tumour fears both my neurologist and my GP said that not only are brain tumours extremely rare in younger people (I was early 30's), having one that presents with just a headache is even rarer. I had horrendous headaches that would have me wishing for death. MRI was clear for anything bad, and even before they scanned me, they were already 99.9% sure there was no tumour. They were only looking for other stuff anyway because I had some other symptoms that went alongside the headaches.
The people who have long running headaches with no scary outcome are so unlikely to write online about them, which is why you'll only ever really see the negative stories (also very very very very rare!)
A stiff jaw is much more likely to be the culprit. I have jaw problems that can cause tension type headaches to last for a month if I pull it the wrong way. Mine is a little bit too hypermobile, so if I put too much strain on it, it can drag at all the muscles around the temples and back of neck. These muscles are also responsible for a huge amount of chronic headaches in people.
Why don't you try some gentle neck and shoulder exercises to see if it helps? If you gentle massage your jaw joint, it might help to ease the pain. Yes tension headaches can last a long time if there's something aggravating them. Anxiety can cause them too, so it could be something of a feedback loop in your case.
Hope you feel better soon