I was a diet coke addict too and I
gained 24 pounds within that time
I am really sure that the diet coke was the cause of the weight gain,because before I started drinking it my weight didn´t change for years,but only two years later I had nearly 25 pounds more on my hips.
Now I am 1,5 years off diet coke and I didn´t gain more weight,in fact I lost 3 or 4 pounds
.
It´s not easy to abandon the habit of drinking it,so I suggest to find a replacement for it.
I started to drink coffee instead of sodas and even if it has a huge amount of caffeine it´s still better than drinking diet coke and stuff.