What can I do about the constant tension in my head that interferes with my ability to think clearly?

I don’t know the exact cause of your tension from your question. It could be mental or physical.

To help, my personal experience when having tension is that for me there can be two main causes: (For you, there may be other causes as well, such as an unknown or known illness showing symptoms which might require a more specialized and specific approach than below. What is below is to give a general idea)

1- Lack of meditativeness

2- Having consumed lately too much of certain types of food like, too much sugar or too much of certain carbs, or certain grains, etc. Basically, foods that depress my energy levels and these depend on my current balance. One food that is good for a person at a certain point might be unbalancing for another person at another point. Basically, this can be better and better understood through long term practice of listening to the intelligence of the body. Each person must listen to their own body and it might also be useful to get help from advanced health practitioners who have actual experience in similar matters to be able to help you.

I try to remedy these two causes by

1- Practicing sitting meditation and taking care of maintaining meditativeness throughout the day. This article can help you introduce yourself to the topic of how to practice meditation. Meditativeness is holding the intention to maintain the focused awareness and maintaining the focused awareness that we learn about during our practice of meditation. When I practice this, I can better connect with my inner intelligence and can live more from this inner intelligence and this enables me to make more intelligent choices, like eating more healthy food. This is because, the subconscious activity of the mind that pulls us to unhealthy habits is reduced when simultaneously we are directing our energy to strengthen our connection with who we truly are and being more of this in our experience. More healthy options and avenues open up as we open our hearts.

2- When I am more meditative and connected with my inner self, I am also more in tune with my body’s intelligence. I make healthier food choices and can be more conservative in how much I eat, I eat more closer to what my body actually needs, and make more conscious choices. I eat less or none of substances like too much chocolate, pasta, yogurt etc, which lightens the load on my body and consequently brain. I can be more in a fast like state, or closer to a fasting state if I am really into a meditative streak. Again, one food that may imbalance a person at a point may help balance another person at another point. One must look at the current picture of the specific person to make intelligent dietary choices.

A few days of diligent meditativeness and careful eating ease most of the negative health symptoms I may experience. In the past it could take a week of pushing out mucus for example and it could be very uncomfortable. The more we take care to eat from nature and less man-made, refined products that are modified and further from natural foods, the better our health will be affected in the long run. I know of really dedicated people who have over decades followed dedicated spiritual practices, healthy diets and detox practices and are in tip-top health and mental peace/clarity and bliss. Here is one article about detox practices from such a person who is also a spiritual teacher. Examine the comments section in the article, too, which contains more valuable insights. This is an example that can give you the idea about the range of this.

If we take care to deepen our spiritual practices (which will lead to spiritual awakenings and in time will lead us to bring us/life energy to our mind/emotions and body to begin fulfilling them) and take care of our body with a healthy diet and adequate exercise, over time we regain our health and possibly even reach great levels of health, depending on where we started at. This approach ensures that we cover the human system at all levels. Clarity is a result of doing what is necessary to create the conditions for clarity to happen. For this we must learn how the mind and the rest of the human system works through direct experience and keep honoring our learning to create the levels of health we desire.