I know what you mean, Root. A lot of prototyping is visual. I've got several versions of a bunch of different parts that didn't look quite right once they were finished and installed... too long/too short, too wide/not wide enough.
The beauty of a scrap metal collection (mountain) is that you can still keep working-- just making smaller and smaller parts.
Switch to carbide end mills and lathe tools like I did; that way, when you break one, you and your pocketbook REALLY feel it.
The trick to being a good machinist is in not believing you're a good machinist. That's when you decide you know it all.
With 55+ machinist's years under my belt, last year I was shown a machining trick I'd never seen before.