M.L.
There's a huge difference between excellence and perfectionism. Much of the difference, I believe, is that "ism" at the end of "perfect."
Ambition is good when it leads you to be interested in life and when it makes you want to set goals to work toward. As the old saying goes, ambition is a poor master but a good servant.
"Perfect" can be an ideal to aim for (i.e., 100% on a math test).
Excellence - if it means doing the best job you can do today - is always something to aim for. If I put in my best effort today, that's excellent. I can try for that tomorrow, too.
A good work ethic means learning to work *at* something and *for* something with energy and integrity. And in itself, it is a good thing.
Perfection-ism is the idea that what I do must be free from mistakes, free from the possibility of criticism by others - and if I don't attain that, my work and I are worthless. This is not good.
Perfectionism, I've found, holds me back because I'll think, "I don't have the time/ability/incentive to clean my house perfectly, so I may as well not do it at all." Or, "I'm not ever going to play the guitar like a professional, so why try to learn?" Or, sadly, "What's the matter with that boy of mine? Can't he do anything right?"
Perfectionism, I found, made me too protective of myself. Anyone could be a threat to my being the best. Even a close friend might turn out to be better, and then I would be "not good enough."
It's taken me quite a while to ditch the "ism" and be happy with excellence - doing the best I (or someone else) can do today. And if it isn't mistake-free, guess what? I don't die!