Corrections in red.  Stylistics and comments in blue.

An [1] Important Change in My Life  [Capitalize the "important" words in a title.  Do not capitalize prepositions]

I used to smoke, but since 2007 I haven't smoked. I think that is a very important change in my life.

I decided to give up smoking, because I got fed up with smoking, and always worried about a place to smoke.  I also felt irritated by the smell after smoking. And I didn't get any pleasure from smoking. So I decided to give it up.

In the first few days after giving up smoking, I worried a little about future difficulties. I thought that it would be very hard, and I would need a lot of willpower. But I was sure the goal was really important for me, and I had to do my best.

Actually, it was hard during first week, but later I've thought about smoking less and less.

Currently I haven't smoked for about six years, and I'm pleased. I hate cigarette smoke, and I feel angry when people smoke near me -- especially in a cafe or restaurant. I finally feel myself free of the annoying habit.

[1]  "the important change" is possible, but only if there has been only one important change in your life.  Because it is probable that there have been more than one important change in your life, I recommended that you use "an" important change.