I have found that aboslute belief is not necessary. What is necessary is that you cease to disbelieve.
Entertain, for a moment, the possibility that your spell could work.
There is a saying that goes: "Your cup must be empty before it can be filled". In order to validate the reality of your spells, try to come to them with an open mind.
As for beauty spells, I haven't tried them myself, but from what I have read it is not possible to make drastic changes to your appearance.
This, however, does not have to do with the DNA-code. Ones appearance change throughout life (most notably during and after puberty), but the DNA-code does not change.