I have problems with concentration and feel stressed a lot lately.what is the reason?what can i do?

Multiple factors: The simple answer is to stop drinking alcohol and smoking, if you do. Coffee and caffeinated sodas should be down to minimal. Decaf coffee still has caffeine so you might try that. Cut your sugar/starch intake. You need to get in the habit of quieting your mind. Meditation and/or yoga are several techniques. Exercise. See a doctor if needed and/or a counselor. You can deal with this. Good luck.

Anxiety: First have a complete physical with a Primary Care Physician to rule out any underlying illness that may be causing these feelings. If you are not ill, then it would be good to consult with a Clinical Psychologist for psychotherapy to feel better and you may want to add medication from a Psychiatrist.

Common problem: People perform at their worst when they have insufficient restorative sleep, are poorly nourished, are fighting illness or in a stressful setting. If some of these apply to or could be improved by you you could see how it helps. If not,meet with your doc and share your history and symptoms. Acquiring these patterns as an adult is more worrisome to me than those that have it from childhood.