Although most people think that 'studying' and 'learning' are synonyms, they are not. There is a very important distinction: 'studying' is the devoted time and activity which may or may not lead to learning, while 'learning' is the actual process of acquiring knowledge or insight.
Learning is good; studying is a necessary evil at best. While both can cause stress, study stress is a greater culprit. What's wrong with studying? The problem is 'study' is confused with 'memorization', which is energy intensive and generally not very efficient compared to an actual hands-on learning experience. Repetition is the mother of learning if we are talking about a physical activity or a complex mental simulation - but not memorization.
The key to reducing study stress is to get to know your own learning processes a bit better, and use your time and mental energy more efficiently. Can you study with a hands on approach? Do that. Make a game out of it. Understand, don't memorize.
Be clear about your goal and use the motivation it gives you. Study stress can easily be converted into a healthy tension from challenge - if you are clear about your goals.