Hi Sammy,
The 3 week mark is one you have not passed before, right?
I'm thinking that this is more about your anxiety over the 3 week mark than about anything physical. You are past the physical detox stage (3 days), so any craves you are experiencing are stemming from behavioral associations/ habits. (Not that the mental can't mess you up pretty good, too.)
At the same time, maybe you can approach this like detox. You know that you just have to get through week 3, one day at a time. Don't bite off big chunks of "Will I be able to keep this quit?" Instead, take small bites of, "I can get through this minute/ 3 minutes (the average length of a crave)/ hour/ day."
And be sure to use all the distractors and substitutes you have been using--have a mint, take a walk, take a nice shower or bath, exercise, take deep breaths, come here to read and post, or do any other thing you have been doing to cope all along.
You will get over this hump. It is entirely possible. And you will look back on this and be grateful that you didn't smoke.
