Hi Pks,
Penile erection is due to blood entering and retaining in sponge like bodies within the penis till there is sexual arousal.
Erectile dysfunction is regular or repeated inability to obtain or maintain an erection.
There are many factors for erectile dysfunction. Smoking is an important cause of erectile dysfunction.
Smoking causes arterial narrowing, resulting in erectile dysfunction.
Other causes of erectile dysfunction are:-
1. Diabetes
2. Some medicines like anti depressants
3. Performance anxiety
It will be helpful if you quit smoking. There are medicines which can cure erectile dysfunction, but they should be taken only under medical supervision as they can be harmful if taken in extra dose than needed.
I hope I am able to answer your query. If you have any doubts, I will be happy to help.
Thanks.

DEAR always mention your age and since how long you r having problem. erection is not a wish and will phenomena it remains hard till you remain excited if excitement vanishes by any cause including ejaculation you r having early ejaculation if before ejaculation then you r getting distracted from sex. a little distraction may diminish erection , concentrate more on pleasure able things and don't judge your penis during having fun may solve your problem if early ejaculator consult sexologist good luck pks.