Inner critic wins... so far

I'm trying to learn about tone in painting and the best way to practice is to do sketches.  The deal is, I think my sketches (this is one of four) really do look pretty good (in a sketchy way), but trying to do the painting, I can't seem to take the chance of screwing it up, and it's tiny, virtually NO RISK.
 Of course, a little failure, like trying to do too much with the sunflower goes such a long way to discourage me.  I'll do it eventually.  I will... now that I've posted about it.  Right?