As someone who's been wearing glasses for all my life:
1. Your eyesight is not exactly "fixed". It can vary during the day depending on stress, tiredness, dryness, etc..
2. A prescription glass can help and does help but it doesn't magically fix your eye sight. If your eyes are strained from lengthy screen usage, the glasses are not going to do anything about that.
3. There is a possibility that your doctor prescribed you un-correct glasses. It took me a while to find my "correct" range.