Anxiety Support Group
Anxiety is a physical condition marked by intense and persistent feelings of distress, fear, angst or dread. General anxiety caused by routine day-to-day stresses usually passes quickly and is experienced by almost everyone at one time or another. However, such feelings that linger over time and are very difficult to cope with, and which lack a clear cause, may indicate...

I would recommend that you go to your eye Dr. so (s)he can take a look and confirm that it is nothing. Also, they can tell you if the black dots you see are floaters. These are not dangerous but are annoying as all hell.
In the meantime, keep the positive talk going so whenever you say "it must be a brain tumor" remind yourself that a brain tumor does not show up over night/instantly. And that it is very unlikely that it is a brain tumor. Keep reassuring yourself that you are healthy and strong.
Take care