It can be due to many causes like psychological fear, fear of non performance, ejaculation dysfunction which can be due to diabetes, high blood pressure, heart or thyroid conditions, poor circulation,
low testosterone, depression
and neurological causes, smoking, muscle weakness, chronic use of cocaine, opioids and amphetamines, diabetes and hypertension.
My sincere advice would be to consult a urologist and get evaluated for these possibilities.
I sincerely hope that helps.