This is a harmless condition called vitreous floater. This usually happens to myopic or shortsighted people as you seem to be. It can happen to anybody and usually only noticeable when you look in bright light or bright white background. It does not require treatment but does not disappear either. Some times it can be associated with retinal tear or detachment or vitreous haemorrhage in which case it can affect vision but if it is long standing like in your case it is not going to cause any problem and you learn to live with it.