Help! Fatigue in week 3

I'm on Day 18 of my quit and things have gone pretty well. I'm past the cravings and, for the most part, I'm really glad that I have finally quit. I don't want to go back and I don't feel the compelling urge to do so.But (there's always a "but") - the fatigue that I am dealing with is making me consider quitting my quit. I am tired all day long - bone tired. Have others gone through this? I expected it, and was prepared for it, early in the quit but it's been nearly 3 weeks of exhaustion and I'm running out of patience. I want to stay quit but there is a part of me that knows that if I just bought that pack of cigs, I could go back to feeling like myself - energetic, focused and ready to take things on. Have others gone through this kind of fatigue this late in a quit? Or is it just me and my options are to live like this forever or go back to killing myself one cig at a time?Help.

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