Quality of a man’s erection

I have no problem getting an erection but my penis goes soft and I cannot penetrate. Why is this and what can I do about it?

When a man is sexually aroused blood rushes into specialized chambers in the penis called corporae cavernosae at such a high pressure and intensity that it chokes off the veins that would normally drain blood out of the penis to create a rigid erection. Anything -like high blood pressure or cigarette smoking-that stifles the flow of blood into or anxiety that accelerates the flow of blood out of these chambers will diminish the quality of a man’s erection.