C.C.
Well... even though it's hard to hear, I think that when men say they are not sure if you're the one, then probably you're not the one. (I'm sorry, I know that is not what you wanted to hear...)
Now, you could be right that it is just the lack of nicotine that is making him cranky. However, let me tell you a little story about a friend of mine. Before she got married, she went on an overseas trip with her fiance. Practically as soon as the plane lands, he tells her that he's not sure if she's really the one because he's not totally attracted to her physically. So for the rest of the trip, they argue over this (and it's not like she can leave because they are halfway around the world from home). Anyway, they did end up getting married, but now she's always thinking, "Does he really find me attractive? Is he just not that into me?" Well, the flip side to this story is that my husband is friends with her now-husband. One night they went out and had a few beers and the husband starts spilling his guts to my hubby. Guess what? He is still not that attracted to my friend physically! After all these years! I think she knows this because they have been in counseling for a long time now and they don't seem very happy together. (And they have two kids together too, which complicates everything.)
Anyway, just think long and hard about how you will feel 10 years from now knowing you may not be "the one," even if you stay together. Are you okay with that? I think you are worth more than that. Why settle for someone who may or may not think you're The One when you can have someone who adores you? (He's out there somewhere!) At this point it is still relatively easy to move on and go your separate ways, but in a few more years, maybe married and with kids together... it would be a different story.
Best wishes with whatever you decide! You'll be in my thoughts.