When you start feeling overwhelmed, give your brain a break. Do something physical; run, swim, ride, or walk. Half an hour of exercise gets the blood pumping and takes your mind off the study. Bonus - it’s good for your body too.
Even taking a shower and belting out your favourite song can refresh your brain.
Sometimes, when you rest your brain, your subconscious takes over, and helps you solve tricky problems.
If you are still feeling stressed after all that, check out this video for some more tips.