Hey Alexx, not passing judgement on you at all, you're body, do what you will of course, but Natvia is an industrial food, a blend of stevia and a type of nectar. It is highly processed and that is never a good thing. You'd probably be far better off with SuCaNat (dehydrated cane juice) or honey as a sweetener, just don't use very much.
Since I started cutting back on sugar back in 07 I've found that I like my coffee now without any sweetener at all, or if I do put any in (very rare) it's a very very slight amount. Usually a gram or less of whatever it is. Same thing with all foods, you really need so little sugar for it to taste sweet, but not if your taste buds are used to sugar overload, exactly the same with salt.
Mmm, time for espresso just now! Have a happy morning everyone!