The basic reason related to problems of erection is lack of proper blood circulation.  When there is a lack of circulation of blood to the penis then there is no nourishment to the structures in the penis and therefore no preparation tends to take place for erection. 

Only half the metabolism tends to work. This is one reason for why when you have the stimulus for intercourse you are left with no erection at all. The reflexes work, but due to lack of stimulus communication to the whole of the structures involved in erection there tends to be a lag.

The reason behind why psychological issues interfere with erection is because when you have psychological panic, anxiety, depression, or any thing that interferes with the flow of energy through your body, you digestive system that processes the nutrients tend to under perform.  This in turn reduces the nutrient supply in to the blood stream, and for some reason blood circulation tends to slow down due to some interference of hormones and pressure.  When there is a lot of pressure in metabolism, then eventually, the erection process does not complete as it should.

You can help yourselves with any of such mess ups with Sildenafil Citrate in eventually enjoying a sustained erection and a pleasurable sexual course without upsetting your partner at all.  Apart from just being on Sildenafil Citrate, look deep in to what should be done to improve your overall wellness of your body.  When the metabolic process is functioning normally, then the rest of it is going to be normal, and your erection issues might reverse.