I am now going to make some corrections using a more colloquial English:

When I was young, my parents didn't allow me to smoke, but I didn't listen to them, and I smoked behind their back.  I smoked for a few years, but I could never tell them the truth.  I was afraid to see the look on my their face when I told them that I smoked.  Finally I decided to stop smoking on my own when I started my university studies.  I didn't feel well at first, but I stuck with it and finally I quit for good. I have taken up sports, and I especially love to swim in the early morning. I know that I made the right decision by quitting to smoke as I feel so much better physically.  Now my favorite activity in my free time is sports!

Well done for quitting smoking!  I used to smoke too, and it's great to feel healthy for your whole life!!

Stop Smoking

When I was young, my parents prohibited to me smoking, but I didn´t listen to them, and I smoked when they didn´t watch to me.
I smoked during some years but I couldn´t face saying the truth to them.
I couldn´t stand watching to my parents´s face if I would say to them the truth.
So I decided to stop smoking when I began my studies degree.
I didn´t mind feeling not very well during some time, but I tried doing it and finally I stop smoking for ever.
Now I have began to do sport, and I love swiming early in the morning.
I think I took a very good decision because now I feel better and I feel like doing sport in my free time

Stop Smoking

When I was young, my parents prohibited to me smoking didn't allow me to smoke, but I didn´t listen to them, and I smoked when they didn´t were not watching to me./and I smoked behind their back. 
I smoked during some for a few years but I couldn´t face saying the truth to bring myself to tell them the truth.
I couldn´t stand watching to my parents´s face if I would say were to tell them the truth.
So I decided to stop smoking when I began attending the university my studies degree.
I didn´t mind not feeling not very well during some time for a while, but I tried doing stuck with it and finally I stopped smoking forever.
Now I have began to do sport excercise., and I love swiming to swim early in the morning.
I think I took made a very good the right decision because now I feel better and I feel like doing sport exercising in my free time.

Stop Smoking

When I was young, my parents didn't permit me to smoke, but I didn´t listen to them, and I smoked when they weren't watching me.
I smoked for some years but I couldn´t face telling them the truth.
I couldn´t stand watching my parents´s face if I  told them the  truth.
So I decided to stop smoking when I began my studies degree.
I didn´t mind feeling not very well  for some time, but I tried  to do  it and finally I stopped smoking for ever.
Now I have begun to do sport, and I love swiming early in the morning.
I think I made  a very good decision because now I feel better and I feel like doing sport in my free time.

Yes of course you made the right decision- well done!

Stop Smoking

When I was young, my parents prohibited me from smoking, but I didn´t listen to them.  I smoked when they weren't watching me.
I smoked for some years, but I couldn´t face saying the truth to them.
I wouldn't have been able to bear watching my parents´s faces if I would had said to them the truth to them.
So I decided to stop smoking when I began my studying for my degree.
I didn´t mind feeling not feeling very well for a while during some time.   I tried doing it, and finally I stopped smoking forever.
Now I have began to exercise do sport.   I love swiming early in the morning.
I think I took made a very good decision because now I feel better and I feel like exercising doing sport in my free time

Aunque se puede decir....say the truth a alguien, normalmente aquí decimos....tell the truth.

En inglés...we don't take decisions....we make them.

Also, we don't do sports here, we exercise or we play sports.