The advice provided by in the initial part of prachigupta's answer is very practical; also, the advice of my colleague will undoubtedly improve how you deal with stress
and studies over the medium to long term.
If you are currently studying for an exam in the near future, then my simple advice would be to focus on the 'NOW' i.e. the present. Too often, we spend time criticising ourselves about how much time we have wasted in the past; or we worry about what will happen in the future if we do badly in the exams. While thinking about the past and future, we lose the present time which is the only time we have for studying and making a difference. So, simply put - try to keep all thoughts aside and focus on the particular subject, chapter or page in front of you. That is all that counts.
Having said that, sometimes the stress can reach very high levels making it difficult to actively focus the mind. In such a case, you would benefit from professional help. If you give more details of how you are feeling, I can advise accordingly.
All the best with your studies!