Magic is in accordance with will. If you cast a spell and do nothing to achieve the effect beyond that, then you aren't putting forth effort. Think of it as a wave you are sending out to case an effect, if you are working at a synergistic wavelength your effect will come out alot better.
Not that I'm saying this is what you Are doing, but if you Were to eat unhealthy and be inactive dispite the spell there would be no results because the energies would be conflicting and self canceling.
If everything we did magicly effected the physical world in a Large way like that, you'd see much more DnD style magics in the world. Magic is a subtle thing, and it works best as an aid in some situations instead of as a treatment.
As well, one needs to have faith in ones self. Absolute faith in ones own spirit and capability (without forgoing the need to continue learning or fostering arrogance) is (imo) extremely important, without loving and having faith in the self one cannot achieve anything. Especially magic.