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I am happy to assist you today.
From what you have described it sounds like the double vision is from an eye turn or tropia of the eye.
Before the eye flat out turns you're likely experiencing symptoms due to a "phoria" which is the tendency for the eye to turn.
A phoria or tropia cannot be easily corrected with glasses. These concerns are treated best via vision therapy.
The goal of vision therapy is to train the muscles of the eyes to work well together to eliminate the doubling effect. I always recommend an eye exam in cases such as what you have described.
Your eye drifts and causes double vision when you are fatigued. This is very consistent with indications for vision therapy.
By performing these training exercises you will likely be able to maintain good binocular vision for much much longer.
Some resources do exist on the internet to help you get a better idea of what vision therapy can do for you.
I suggest starting at this webpagehttp://optometry.osu.edu/research/CITT/index.cfm
Some vision therapy equipment can be obtained online. I suggest starting with a Brock string which I've seen on Amazon and Ebay.
I'm more than happy to answer any follow-up concerns you may have. Please don't hesitate to contact me again.
Please don't forget to offer a positive rating once your question has been answered fully.Also, please note that the information here does not replace an examination and should be used as educational purposes only.