Just endure. When you get animate dead and other mid level spells, mages become way better. Also, play smart on low levels. For example, exploiter wizards can use powerful exploits and will not run out of spells as quickly as another low level magic user. Instead of fireballs and other offensive spells, buff your party members. You will gonna be way more useful casting mirror image in your self, enlarge person in your companion, blur, etc.
Also, turn off XP sharing and hire a merc. I'm playing as a Cruomancer and reached tier 8 magic in chapter 2 by merging my spellbook with Lich.
Yeah, currently my mage is just a greasy grease flinger. I know he will get better, but say in BG a low level mage gets carried by others, here I feel the other party members just suck. The only semi-reliable one is the black female paladin, but one crit and she is history too.
And I'm on that defense mission. Jesus wept. Compared to my paladin from the beta, it's like night and day for some reason.