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With astigmatism one can have few and mild symptoms like little blurred vision, eye strain and mild headache.In your case it does not seem to be the reason.
As you say, your symptoms are present for quite some time, with inability to concentrate, feeling fatigued and yawning most of the time, I would like to say perhaps you have some busy daily life schedule/life style in which you do not get enough time to have a better sleep and leisure time to spend and relax.
But being a a medical professional I would like to rule out any underlying illness for theses symptoms like "hypothyroidism", a disease in which one's body metabolism gets slowed down,patient feels fatigued,lethargic, becomes slow, can not perform daily activities with full strength, there is weight gain, gradual change in voice (hoarseness), high blood cholesterol level and later stages lead to problems with blood pressure and cardiac symptoms and fluid accumulation in body spaces.
I would like you to get your basic tests done including CBC (complete blood count), Serum TSH (thyroid stimulating hormone) level, serum lipid profile in fasting state, creatinine (kidney functions) and ALT level.
If everything turns out to be normal, you may need to look around your life style which may need little modification including healthy food intake,moderation of alcohol consumption if you use it, reducing cigarette smoking if you smoke and daily 30 minutes brisk walk. It would be extremely beneficial.
I hope you find my opinion useful. If you have any more queries.you are most welcome.