the focus of mana boost just saps my mana? i put it on, the mana bar shrinks, and it dwindles. what the hell is up with this debauchery?

it reduces your mana regen, if your regen is too low this happens.
I beleive the worn notebook entry warns you about this but I'll add it to the item description

however, that behavior is in fact intended. Better gear will help.