Why does my left eye droop? (Photo)

A few years ago I noticed my left eye start to droop and now it is noticeable in my ID photos and in person, I had an accident to the front left of the top of my head where I cracked my skull and got stitches, I have worn part-time glasses for the past 4 years, and have an astigmatism in my left eye. Could the cause be one of these or something else and how do I fix it. Please help.