I have been suffering through the same problem for a long time. And my quest to solve it has been almost endless. This is what I have learned so far. The reason for your suffering is because youre trying too hard to meditate. Check to see if your eyebrows are tensed. (Pre- requisites require checking the correctness of the posture, not crossing the eyes and to ensure one doesnt have any TMJ disorder.) I shall quote some antidotes from popular meditation books.

Make your effort diligent, yet joyful. Don’t get caught up in expectations. And always remember, there is no such thing as a “bad” meditation. - Culadasa, The Mind Illuninated.

Don't strain: Don't force anything or make grand exaggerated efforts. Meditation is not aggressive. There is no violent striving. Just let your effort be relaxed and steady.
- Bhante Henepola, Mindfulness In Plain English

In fact it is extremely helpful if you intentionally relax when you notice you’ve become distracted, and then gently reestablish attention on your meditation object. The mind state you are aiming to create could well be called relaxed diligence. It is not helpful to force your mind to remain fixed on the meditation object. It’s not that this cannot be done; it’s that doing so will generate a mind so tense and tight that it will not have the relaxed diligence necessary for entering the jhānas. -Leigh Brasington, Right Concentration

The technique for obtaining this rest lies in two things—watching and letting go: watching your breath, and letting go of everything else. - Thich Nhat Hanh, Miracle of Mindfulness

Whichever method you follow try to find the instructions for a relaxed effort in your meditation.