It is usual for a young man to have a psychological erectile dysfunction rather than a physical cause. In your case chronic alcohol addiction seems to be the culprit. In a recent study, it was analyzed that 60% of chronicalcoholic men are likely to suffer from sexual dysfunction. Chronic and excessive intake of alcohol may notably affect the Nervous system and organs like Liver, Pancreas etc. This results in memory disturbances, concentrating abilities and all other symptoms related to Autonomic nervous system , leading to decreased Libido.
During an erection, the penis is filled with blood and the valves are closed to prevent the backflow .This contributes to the erect stature of penis. Excessive alcohol consumption over months, damages the valves in the blood vessels leading to retrograde (backflow) flow of blood. This is expressed as inability to maintain erection for a long time. Alcohol addiction over years leads to atherosclerosis, a condition where there is accumulation of fatty materials like cholesterol in the walls of the blood vessels, leading to decreased filling( because of narrowed vessels). This again results in difficulty maintaining erection.
I would like to recommend you the following:
1) A general physical assessment along with hormonal and nutritional status
2) Liver function tests.
3) Try to Taper the consumption of alcohol to as little as possible and avoid its consumption before and after sex.
4) Take Vitamin supplements like Vi B 12,as alcoholics are deficient in B 12 because of decreased absorption.
5) As this condition is always associated with part of psychological stress, psychosexual counseling for you and your partner would be of great help.