Semen production goes on in the testis for 24 hours and gets stored in seminal vesicles. Now imagine seminal vesicles as a water tank. 24 hours production at some time, will make tank full. Its storage capacity runs out. Now overflowing mechanisms take place like night falls and seminal discharge in urine. To avoid this u need to have sex or masturbate regularly. For premature ejaculation u can try masturbating 3-4 hours before sex. As when tank is full, slight sexual activity will lead to ejaculation, if u release some pressure earlier, u will last long. If u are overthinking about ur ability to satisfy your partner, u need anxiolytic medicine. Consult a Psychiatrist who delas with psychosexual disorders. U can also consult me through practo chat.