You are noticing vitreous floaters.
The vitreous is the gel that fills the posterior 2/3 or so of your eyeball.
It varies from person to person but most people normally have some amount of cellular debris that is suspended within the vitreous. As light enters your eye it passes through the vitreous until it reaches the retina.
If you happen to have debris suspended in just the right place, this debris will cast a shadow on the retina which you see as a “squiggly black line”. When you move your eye in an attempt to center the shadow, the gel in the eye will lag for just a moment and then will rotate along with the rotation of the eye.
This returns the debris more or less to it’s original position. That is why the shadow seems to run away from you as you attempt to move your eye to look at it. Frustrating? Yes. But it is seldom an indication of any dangerous eye disease.
Rarely, though, this debris can be deposited in the vitreous due to a tear or detachment of the retina.
Should this be the case, surgical intervention is needed.
The fact that your floaters appear to be long standing and not really changing over time leads me to believe that you have regular old vitreous floaters that are more a frustration than any real problem.
To know for sure, though, a dilated exam of the eye is necessary. So to set your mind at ease why don’t you take the time to go in and let an optometrist look at you. Hope this helped.