Hi Allie.
I feel your pain. I was addicted to the stuff too. I drank it every damn day, and if I missed even a single day then I got brutal headaches.
I can't say that this will work for you, but here's how I did it.
I quit, cold turkey, just like that.
Well, almost. I knew I was going to have problems with the caffeine withdrawal, so I switched to iced tea (real iced tea, made with teabags and stuff, only added 1 tsp of sugar per 2 litre batch, but also added a bag of peppermint tea to each batch). I then took that and cut it even further with water as I drank it. I would use a 1 litre bottle, fill it with iced tea, and as I drank it down I would keep adding water until it was very weak, under 1/4 strength.
A couple of weeks of that and I was onto just plain water. No cola. No tea. No diet cola. No headaches. No fatigue.
In the last six months, I have had cola about 3 times, and each time I threw half of it away. I just don't like the stuff anymore.
I work hard at not drinking the diet stuff, both because it tastes vile and I believe that the stuff is bad for you. I do, however, drink it a little. The city water in my town is so awful that diet cola tastes good by comparison, so at a local restaurant I will have a diet pepsi. That doesn't happen very often, and when I am in other cities I cheerfully order water.
It can be done. I did it, you can do it.
I wish you success in getting that monkey off your back.