The spell mystic shield says
This spell creates an invisible aura around the recepient that moves with that individual. It prevents all spells and spell-like abilities of 6th level or lower from affecting the recipient. The protected individual can cast spells normally, even on himself. (Anauroch: Empire of Shade 141)
Can a caster who's affected by mystic shield cast spells when he would've been otherwise affected by the spell antimagic field when that spell's a 6th-level or lower spell?
Let's take a look at what AMF actually does:
The space within this barrier is impervious to most magical effects, including spells, spell-like abilities, and supernatural abilities. ...
An antimagic field suppresses any spell or magical effect used within, brought into, or cast into the area, but does not dispel it. ...
So spells can't affect anything inside the field, and magic cast out of or into the field is suppressed.
You yourself are not affected, just your spell effects. You can still cast inside the field, with or without the mystic shield, but as soon as your spell is cast and turns from mumbo-jumbo into a spell effect, it's no longer part of you, and becomes suppressed.
However, you still have all your buffs, since having your buffs suppressed is most certainly affecting you.
The bit about being able to cast spells normally doesn't help you, since your spells are suppressed as soon as they are cast.
