I don't know why it is coming out louder in iTunes than GB,
iTunes has the optional Equalizer. Try turning the Equalizer off or set it to 'flat'.
To me, the most difficult part of the process is the mix. What I have done is to set up my eq setting in iTunes to give the best sound on a professional recording. In other words, take a recording that sounds well done and use it as your standard for your eq setting in iTunes. Then, when you mix down your Garageband session adjust the Master track eq until it sounds good in iTunes. If it is too heavy on the bass take a little bass off. If the treble clips, take some treble off. If it is too harsh work on the mid range. I find Garaband's simple EQ effect quite useful for mixing, especially the mid range slider.
I also listen to my mix on my boombox, car stereo and iTunes and keep fine tuning it until it sounds good on all of them.
Turning the mix down will avoid clipping, which is a very nasty sound in digital recording, but you will lose more quality when Garageband mixes down to 16 bit. The trick is to keep the mix high without clipping.
Kurt