hi, You seem to be suffering from erectile dysfunction. It means inability to achieve sustained erection with enough rigidity for intercourse. Severity varies. You seem to able to achieve an erection but not able to maintain it and also rigidity may not be good enough. Most of the erectile dysfunction is pychological. However if you are a smoker, stop smoking. Exercise regularly. Evaluate for blood lipid profile. Get an ECG done. Sometimes it could be because of some vascular problem. Most important thing is regain your confidence. consult an urologist.