To use magic you consume your magic power; which is why you feel weak after using too much.
As your magic power depletes, your whole body starts to feel sluggish – if you continue to use magic then you end up fainting. Training until exhaustion during childhood will increase mana capacity and skill; if you haven’t reached your maximum potential by the time you’re an adult then it is near impossible to do so as training once reaching adulthood has a significantly reduced effect.