Impotent

Sometimes, men are difficult to keep the erection. It is usually because of stress, tired, alcohol, and some diseases. Temporal impotent is not a big problem if they still can erection on masturbation or wake up with erection. Impotent in long period could be happens in diabetes, blood circulation disturbance, and consuming diuretic and antidepressant drugs. 
The things you can do are:
1. Don’t judge yourself. If you are stress or tired because of finance, work, or disease problem, manage them first.
2. Tell it with your partner. Talk about your anxiety and try another variety of sexual intercourse to reduce your anxiety.
3. Do some breathing exercise and muscle relaxation.
4. Reduce alcohol, better if we stop it.
5. Stop smoking. Men with smoking habit have erection risk twice than none.
Practical technique you can do to decrease your sexual anxiety is to be relax with your partner, some foreplay like flatter and touch (without touching sex organ) can help.