Liza,
Congratulate yourself on 18 days nicotine free! You have no nicotine whatsoever in your system and you are healing. I've been free over 6 weeks, it does get easier. I also had rough times occasionally but once you get through them it is well worth it. Think of it this way - you can be an ex-smoker occasionally wishing you could take that one cigarette, or a smoker constantly wishing you didn't take that one cigarette. The urges do get less drastic and fewer, I can promise you that.
One thing I think about when I get the urges is all the reasons I don't want to smoke, such as paying over $5 a pack, or standing outside alone to get my fix, etc.
Just remember, NTAP, one day at a time.
Jeff