My Left Eye Started Sinking into my Eye Socket Years Ago & It Looks Like Its Sunken in More Since. What is Causing This? (photo)

It is difficult to tell from your 2D straight photos but you possibly have enophthalmos.  If this was acquired without any history of trauma, it could relate to a condition called "silent sinus syndrome".  Please obtain a complete oculoplastic exam from an oculoplastic surgeon.

From your submitted photos, you don't appear to have enophthalmos, which is retropositioning of the globe within the orbit.  The most common cause of enophthalmos is post-traumatic.  Your appearance may be slowly changing for another reason however.  See a qualified professional in your area for examination, and likely imaging studies (CT scan, MRI, or other).

Based on your photo, it is hard to tell if you have anything wrong.  A top down photo may be helpful.  Regardless, diagnosing enophthalmos (sunken eye) needs to be done by an oculoplastic surgeon.  

In my opinion based upon the two posted photos you have a familial, genetic family very slight asymmetry. As for definitive diagnosis of enopthalmos might be a stretch. Best to be seen in person by boarded PSs in your city. If indicated and you can afford the 3-D scan of your orbital anatomy you can pre sue the definitive diagnosis. Otherwise try a filler or fat grafting or even minor surgery to attain better symmetry. Again the degree of asymmetry is very minor in my opinion based on the info and photos posted.