I'm relaxed and remember to breathe when I ride, but my back, especially my lower back, is always really tense. The only time, recently, that it has been loose was the other day when we were practicing adjusting our strides from a slow canter to a hand gallop.

So what causes your back to get tense? And whats the best way to deal with it?

There is no magic cure for a tense rider, or a tense back, specifically, but it certainly helps to stretch before and after your ride. Try not to focus too much on your back as constantly thinking about relaxing it will often have the adverse effect. I would recommend doing exercises on the ground to help strengthen your back, as the tension is often a side effect of weak muscles. Keep in mind that a lot of riding 'comfortably' has to do with muscle memory, and it doesn't come all at once. Just keep yourself healthy and keep practicing.

Best of luck...