I agree with the first two replies. Just stick with the regular Coke with your Jack. You are making great progress, although it may seem a bit slow to you now.

A good target for most people when trying to lose weight is about a pound per week. This is because to achieve that rate, you have to have a deficit of 500 calories compared to your daily requirement, every day. The reason is that it is hard for people to cut this much through diet and exercise, at least initially, because they are probably consuming more than their requirement, making to necessary cut quite large. You are nearly doubling that rate, which is excellent, and doing it using both dietary changes and Exercise, which is the only way that has been proven to work.

People who are successful in the long term with weight loss do not deprive themselves of all the things they love, they simply moderate their intake, and allow themselves a few indulgences. If Jack&Coke is yours, then i say go for it. If you really feel guilty, spend a bit of extra time on the treadmill, or increase the incline or pace. The amount of calories you would save by switching to diet (per drink) is approximately the equivalent of walking a mile on flat ground at a brisk pace (less on an incline).

Sorry for being long-winded, but the point is keep your reward. It will help you keep up your motivation and be successful both losing weight now and keeping it off in the future.