Barring any physiological causes, I'd say it's probably performance anxiety, which is common. Psychological causes such as stress, anxiety, guilt, depression, low self-esteem, and pressure from a new partner can result in erection problems. Anxiety over one's ability/inability to maintain erections creates additional pressure and may contribute to the problem.

To diminish the problem, I'd avoid smoking (reduces blood flow to penis), keep your weight in a healthy range, and avoid drinking alcohol in excess as well as avoiding illegal drugs (both restrict blood flow to the penis). If you are having a problem managing stress, you might want to address that, too.

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