The effects of caffeine vary widly between people and the particular preparation. You may just have a high natural tolerance, L.Spiro
My girlfriend becomes extremely excitable, irritable and head-ached from any amount of coffee, but she's relaxed by chocolate. It's not likely just placebo, as she'll break out in hot flushes and sweats, as well as the behavioural changes, even from desserts where she didn't know there was coffee in them.
Personally, I can feel espresso coffee "working" in my brain (like a very mild nicotine rush), but I've never gotten the same effect from regular soft-drinks or chocolate.
If I do have a block of chocolate at my desk, I can stay up for long periods without losing too much focus by eating it over the course of the night. That's not an intentional thing, more of an accident where you suddenly realize "oh crap, it's 4am and I've eaten that whole block".
With Instant-coffee or "energy-drinks", my brain feels "tight", like it doesn't fit in my skill, and I feel tired soon after.
With any form of caffeine, I know I've had too much when I get dehydrated -- when you get the urge to drink a litre of water but are still thirsty afterwards, your eyes dry out, etc... The main cause of hangover symptoms is also dehydration, so it makes sense that you can feel 'hungover' from drinking too much strong coffee.
Like I said above, instant-coffee and real coffee are almost completely different drinks. Also America is renowned for their love of terrible drip coffee, so there may be a different preparation that you do like. I personally didn't really like it that much until I moved to Melbourne where there's a good culture surrounding it (and a lot of coffee snobs). The method that's used to extract the coffee from the beans (the grind, the temperature, the pressure, etc) has a huge impact on the chemical composition of the resulting liquid.
It's like hating all pasta dishes, because you had a seafood pasta once that you didn't like