The trick with a hobby knife is to scrape, not cut. Keep the blade perpendicular to the surface, or even angled a little backward to the direction of movement, and use very little pressure. It takes time because it'll take off very little at a time, but that's to the good when you're not yet comfortable with it. As you get more comfortable, you'll just naturally move into more direct techniques.
Oh, yeah, and very nice job. I've seen people that have painted for years do worse.