As you already stated, you have no significant medical problems, which helps to put many causes much farther down the list of possibilities.
I believe your symptoms and decreasing vision could all be caused by an increase in the cataract previously noted in your left eye. It is not uncommon for one eye to have more of a cataract then the other. Cataracts tend to cause more and more of a myopic shift, difficulty with night vision and, in some types, increasing glare. In my experience I've had patients with cataracts who noticed changes in their eye glass prescription every three months prior to having cataract surgery.
When it comes to important issues I always believe in obtaining a second opinion. Any Board Certified Ophthalmologist in your area should be able to diagnose your condition and discuss possible treatment options.
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