Don't worry about it, they're all terror stories. As far as I've heard, you can transfer 2 litre of blood every time you donate. And a litre is close to 2 pints. So there's no worry about blood loss.
If you get dizzy, it's rather your fright that gets you than the actual blood loss. Especially if you're a large person (in any way). Just remember to eat and drink in good time before going, it will keep your blood sugar up and your brain powered.
I would talk to the nurse about this, to hear a professional opinion just in case though. They know best.
And as to the pain, it's all about your mind set. If you're scared, you tense up, it hurts more. Adrenaline is also a natural painkiller so there's nothing wrong with being nervous.