I never had this problem, even though my wrist curved back when I used to aim with my wrist (I use my arm more now).
I'm guessing that you used to be a mouse player and you're kinda new to tablet. The wrist curving doesn't effect mouse aim because the mouse curves at the same time with the wrist, while the tablet doesn't.
I think you should try focussing on the cursor location instead of how you move your wrist and fingers if that's what you've been doing. You probably just have to play until it becomes muscle memory.
Another thing you could try is using your arm more when you aim. If your wrist stays in one position on the table, you can still kinda wiggle your arm side to side in the same direction you move your wrist. Maybe that'll help with drawing straighter lines.
Maybe you could try making your area smaller as well, the curve would have a lesser effect over a smaller distance.