Problem with straightening ankle

Hi there,

All my life (I think) I haven't been able to fully straighten my ankles, in line with my leg, the same way everyone else seems to be able to. This hasn't really been a problem, and explains why I'm good at long distance running but not sprinting, but I've just started kickboxing training and it seems to be causing problems. Since I can't straighten my ankle I can't kick flat surfaces (kick pads etc) without it really hurting the ankle, or hitting my toes (which should be pointing the same direction as my legs). I know you're supposed to kick with your shin, but its hard when the ankle isn't in line.

Anyway, I was wondering if this is likely to be something I'll be stuck with forever, or if theres certain things I can do to increase ankle flexibility?

Thanks in advance for any help!