I had the exact same problem with the circle pad for years - it worked, but it would often get "stuck" in one direction (usually the right). Finally got round to fixing it around December - some people on eBay sell spart parts for 3DS', IIRC circle pads are fairly cheap on there, and there are several tutorials on how to replace them, if you're brave enough to fix it yourself. (My mum fixed mine - I'm no good with tiny fiddly things like that) Not too expensive and didn't take too long to fix, either.
I hope yours can be fixed!