Stop stressing. Honestly.
One of the whole things about doing fanart is to do your own personal rendition of the characters! c:

personally i think it's one of the charms of doing fanart to be drawing them in your own style. it gives them a bit of spice/variety and makes it individualistic.

Let it all out. Have fun. Regret nothing! =3

go for it! no matter what your style is, it's fan art, the point isn't really to make it look exactly the same style or similar to the original style, it's just to have fun and express personal headcannons and things like that, and peoples personal styles makes the art look all the more cool and fun!