A tablet will make it much more intuitive. I'd recommend giving Paint Tool Sai a shot; the way the brushes handle in that program is incredibly intuitive. Open Canvas is another cool little program, but the way the "paint" handles isn't quite as easy to understand, imo.
Your pixel art avatar is incredible looking, with all that fantastic sculpting and the great color choices. You're obviously a good artist, so I think you must just be overthinking things here - in your painting, you used no hue shifting, and used black lines to separate the different forms. On top of that, the forms are all much better defined in your sprite version. Drawing is drawing. Since you don't have the benefits of pressure sensitivity, why not just use a brush that's nearly opaque? Just do that, and draw the same way you'd make pixel art. The results will be great once you get a little used to it, I'm sure! =)
And again, try Paint Tool Sai. For digital painting, finding the right tool for you makes a much bigger difference than in pixel art, since all the programs behave so differently. Paint Tool Sai is the easiest program I've found. Good luck!
You could also take your sprite, blow it up, and paint on top of it. Or scan in a sketch, and paint on top of that. Drawing so large with a mouse is very difficult; having the basic sketch down to paint on top of will help a lot, probably.
Can't wait to see what you make! Pixel artists are all secretly incredible painters on the inside, picking every single color deliberately, without the crutch of being able to smear paint around like normal painters. Just draw the way you always do. =)
Edit: Ah, also, Open Canvas has a nifty feature that saves every single brush stroke you make, for later playback. You can then upload it to their website.
http://www.portalgraphics.net/pg/ Check them out if you want. It'll give you a good idea how to make certain effects.
Remember, though...you always have the option of using a completely opaque brush, and doing very large pixel art, hahah. It's an excellent skill to have!
