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My dear, do not worry. Don't be anxious. You will do fine with your girlfriend. Learn to engage in foreplay. Talk romantic things with her, cuddle more, explore wioth your hands more. All this leads to lot of adequate sexual stimulation which will help you get a firm erection. Most of the time the cause is psychological due to low confidence like premature ejaculation
. Learn to relax. Do deep breathing exercises. Do Yoga or meditation
. Sleep for 6-8 hours a day. If the problem does not subside, you may visit a Urologist
to help you. Masturbation
does not have any adverse effects, don't worry. Have faith.