Brief Answer:
Most likley due to anxiety.

Detailed Answer:
Hi

Thank you for asking HCM

I have gone through your query. your problem can be most likely due to anxiety. During masturbation lasting less than one minute is normal. Because your urge is for pleasure and ejaculation and also there is no natural stimulus.
Low testosterone levels also can cause decreased erection and decrease in size. By doing regular exercise daily and taking nutritious food you can overcome this. Body building exercises is good for increasing body's natural testosterone also. Taking aphrodisiac foods like sea foods, egg , meat , nuts and citrus fruits is good also.
Avoid tight underwear. Use cotton and loose ones. Avoid underwear while sleeping. Avoid using hot water on genital area also.

Let me know if anything not clear.
I am happy to help you.
Thanks.

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Brief Answer:
Testosterone level should be checked.

Detailed Answer:
Welcome again.

Thank you for your follow up query.

Do you have normal morning erections when you getting up from sleep. ?. If you have that then it can be most likely psychological and in that case you should get a behavioural therapy by psychologist.
If there is no early morning erection then check your testosterone level in your blood. Also get a consultation with urologist to check your function of testes. Ultrasound examination of penis also will be needed. If there is no organic problems and low testosterone then natural methods like exercises and nutritious food intake can be tried. If it is not improving then testosterone supplementation will be helpful.

Let me know if anything not clear.
I am happy to help you.
Thanks.

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