I've had the same experience with caffeine- I am very addicted and without it the withdrawal includes a bad headache, serious fatigue, and not being able to think about much else except wanting caffeine. Yet lots of people have told me with a straight face that I am "wrong" and caffeine is not addictive.

Interesting, I also have the same symptoms as you, including insomnia even if I limit coffee to early AM. I think these effects are characteristic of "slow caffeine metabolizers," e.g. people for whom caffeine has an unusually long half life. My hypothesis is that for these people blood caffeine levels stay relatively high 24/7, so the body never gets to adapt to functioning without it, making addiction more likely.

It's weird to me that people seem adverse to the idea that peoples bodies and genetics vary, and one persons experience isn't going to be the same as another.