Caution, you asked for it, so I'm going to give you straight junkie thinking without censorship:
1. My biggest fear: Probably a tie between
A. I won't make it -- I've been a nicotine addict for almost eighteen years, and I failed my first four serious quit attempts, all in the first four years of the addiction, and I had advertised all those quits to family, friends, etc. So I spent fourteen years addicted with no quit attempts because I was afraid I wouldn't make it (and deluding myself that I could postpone quitting forever by using Nicorette for eleven years).
B. I won't be able to concentrate at my stressful highly demanding job, so I'll lose the job, and in the resulting stress, be forced to pick up nicotine again.
Roll A and B back together into one fear: My case is different, other people may quit or never pick up in the first place, but unlike them, I really, really need nicotine to cope with life, where maybe they don't. (Sorry, just trying to put junkie thinking in words).
2. How'd I overcome it:
Education and support. Reading whyquit.com over and over again. Learning that I was a fool to think that I could just keep on indefinitely chewing Nicorette, dipping snuff, and sneaking the occasional cigarette and cigar. Learning that in 72 hours, I would be nicotine free and on my way to comfort. (Shoot, somebody told me that in law school and I ignored them). I guess I was ready to hear it and the group support is great too, proving it can be done.
3. What did I learn in the process: that I am ready, ready, ready, to put nicotine in the rear view mirror by not using it in any form from day to day. Ready to get healthy, quit wasting money, be calm, sleep better, and look better. I just did some calculation: I don't know exactly the day I got addicted, but I was between 17 years and 8-10 months old. After that, until this seventeen day quit I am on now, I was hooked on nicotine for between 17 years and 8-10 months. Give or take a month or two, my life until October 18, 2003 splits about in half; half non-nicotine addict, half nicotine addict. If I stop the clock now, I will only add clean time to my life and I'll get to spend more of my life clean than addicted. If I could get through seventeen years and change without nicotine before, (no matter what stress I was under), I can do it again and more, if I never ingest nicotine again.