The reasons for difficulty in maintaining erection can be many such as Hypertension, Diabetes mellitus
of the arteries, Neurological problems, Chronic illness, Prolonged intake of drugs, Stress, Smoking, Alcohol, etc.
So, investigations like Complete blood count
, estimation of blood pressure & blood sugar levels, lipid profile
will be helpful to rule out any pathological cause for your symptoms.
Other than that, you need to make certain modifications in your diet & lifestyle:
1. Have a diet rich in fruits, vegetables, fish, nuts & honey.
2. Avoid fast foods & lipid containing diet.
3. Avoid smoking & alcohol
4. Keep away stress, think positive.
5. Exercise regularly & practice meditation
One more thing...avoid consuming any over the counter medication
which claim to improve performance...it may be harmful.
Wishing you a good health.