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.