Hi, I Am 28 Years Old, I Am Married Since

Hi, I am 28 years old, I am married since 6 months. I am suffering from erectile dysfunction since a long time. When i do sex the erection comes rarely and when it comes it goes down as soon as i start sex keeping my partner unsatisfied, also i feel that the sperm coming out is not mature. i consulted a doctor and shown my problem than he had given me tazzle 20 as a prescription for 15 days. So as soon as i take the medicine the erection kind of feeling comes within 1-2 hours but i feel it just works like a viagra and as soon as the effect of medicine reduces the same problem arises i.e. problem is solved temporarily. So I can't take it always. I want to know what is the actual problem with me, why is the erection not there without taking tablet. what all tests can i do to know completely the reasons behind my problem ? What all tests related to sex i can do so that i can move further with proper treatment for this problem ? Is there any deficiency in my body which is resulting into this problem ?