A good hand brushed finish is a real art. Even though airbrushing takes some practice, I really believe to get a good hand brush paint finish on an entire model requires more skill and practice than an airbrush. So do a lot of practice on scrap plastic before you try again, after you get old paint off.
I am sure you'll get a lot of responses- there are a number of things that will remove paint, everyone seems to have their own favorite. One thing that is not frequently mentioned, that works for things like modern jets without a whole lot of fine surface detail is very fine wet or dry sandpaper- say 600 grit. A lot of work, but it does get rid of paint. You do not need to get every bit off, just get it down to a smooth surface with no runs or spots, then apply again. Do NOT use paint thinner. BTW, you did not say what kind of paint you used.