Just practising isn't enough. You also have to practice.
What I mean is, a little, or even a moderate amount of practice may not show any results. But through repetition, adjustment and genuine effort, you'll eventually get there. I have to tell that the technique for both feet is identical, if you hadn't realized. It's just a matter of muscular strength, balance and of course, mental preparedness and confidence.
Let me relate to you my personal story of achievement. When I was about 12 or 13, I got deep-cleated by a horrific challenge in May and it screwed up my right leg pretty bad. I was a terrible left foot shot, and didn't know what to do. I wasn't able to hit shots with my right for sure, even though I could run on it and stuff. Summer was coming up, and I wanted to get in a lot of practice because I had plenty of time to devote to it.
So what I did was, living as close to a field as I do, I would go over like, three times a day for an hour, and only hit left footed deadballs. From all sorts of different areas. It was terrible at first, but, as I only used the left foot exclusively, after weeks, I noticed a great improvement. When I was finally able to use my right again, I found I prefered the left! The right was still decent, and after practicing with it again for a little, it was pretty much back to where it was before the injury. But because of my effort, I was now endowed with the footballer's greatest weapon: two functional feet! I can use the right to make space for my left, and vice versa, depending on the situation. It feels great to have that option.
That being said, if I was in a pinch, or had to pick a foot... I still choose the left.
The point of my anecdote is that you too can achieve what I did, even as a natural righty. I still struggled with for sometime after, and do a little today, to pull off finer things like difficult tricks with the left, that come more easily to the right. But my left and right shots are both great, and the left is likely to be even a little better. Because I saw the results myself, I had no qualms in taking shots with the left, and I have total confidence in it. Keep practicing and when you improve, the confidence in your left will allow you to hit great shots with it!