Hey!
Excuse the length of this reply! I feel the need to justify my potential suggestion so will reference my experience with the same symptoms.
This is 100% relatable. I've had the same issues for over 2 years (and it kept getting worse for a significant amount of time!)
I went down the same thought process as you - "it must be a brain tumour! I have some fatal disease! I'm going to lose my vision!"
My eyesight got so bad that I've had to stop driving and my quality of life is minimal. It's truly terrifying when you get double vision even when wearing glasses. Everyday is just a struggle - though not having answers triggered anxiety and depression - which can manifest their own symptoms.
I had 4D scans of my eyes done privately, I had brain scans, saw a countless numbers of Drs with various specialties - no answers. (other than, "anxiety" - though that's a chicken/egg issue)
In December, I had to see the Neurology and ENT department in the same day. The Neurologist basically said - there's nothing wrong with you, learn to live with it. I felt like I'd been given the death sentence.Anyway, a few hours later I went to my ENT appointment (regarding my tinnitus, specifically, pulsating tinnitus, though I also have high pitched tinnitus too)
My appointment was with a clinical scientist, who saw how distressed I was. I started to explain my issues to her and within a few minutes she was finishing my sentences for me. She knew exactly what it was (generally speaking); a vestibular ocular reflex disorder. Google it. You may find it scary how much of the list of symptoms relate to you.
To summarise "vestibular ocular reflex" though, if you look straight ahead and lock your eye sight on something in front of you, then turn your head slowly to the left, you'll notice your eyes move to the right, and vice versa. That's the basics of the reflex.
In my particular case, due to severe inner ear damage - my vestibular ocular reflex is affected! Causing me have the same symptoms as you (and multiple other symptoms which seemingly would be unrelated)
Each case will be different so I won't go into the inner details of my particular experience - however I thoroughly recommend going to an ENT department as ask them specifically about this! As mentioned earlier, I saw numerous healthcare professionals and had no luck. Even the specialists I saw didn't make the connection. I was convinced it was a brain tumour. It was by a miracle I met that scientist who recognised my symptoms which no one else had linked together before.
I'm not having any treatment for it yet, as it's a long waiting list. There are more sub-categories of vestibular ocular reflex disorders apparently, but I won't have the specifics until I get booked in to see the Dr who specifically treats people with this issue. - but simply knowing it wasn't a brain tumour and having the anxiety removed made my life more bearable.
Obviously, you may not have this as I'm not a Dr nor qualified to diagnose - but I specifically joined this forum just so I can reply to your message as I know how helpless and scared eye symptoms can make you feel - and I hope you manage to get the help you need soon!
One final note, would be to make notes of when it gets worse. My eye issues spiked when looking down aisles in supermarkets, driving at night, looking at certain types of monitors etc. Having a record of it's frequency, and any potential triggers will be very helpful for a diagnosis IF it is that.
All the best, and keep me posted