Will aspertame ruin my diet?

No, it won't ruin your diet, but it causes headaches for a lot of people, and it just plain tastes bad. It has a really bitter aftertaste. If you're trying to get healthy, unsweetened green iced tea is good for you, and you don't have to worry about the effects that chemical sweeteners will have on your body in the long run. If you just can't get used to the idea of unsweetened tea (it really helps if it's not brewed too strong) try stevia as a sweetener.