Black dot in vision. What i should do?

I see black dot whenever see, in my left eye. I have been wearing specs for 6 years. The vision power in the left eye gradually decreasing. I could feel it.When i focused on seeing an object, the right eye lies with the object and left eye ball slightly moves other side of the object(the people told me this) . I don't know how its been called.I doubt this might be a reason for the appearance of black dot. I have this problem for about 5 years. I have diagnosed for it before 2 years the doctors told its possible to cure it through eye exercises and later they told that i should under go an operation. But i didn't go for an operation. Now i fear that i might loss my vision power. Suggest me what i should do??