I don't think it's confidence, or at least not entirely. A few things are happening here.
1) He's not driving to the bucket anymore. Therefore, defenses don't need to respect his ability to put the ball on the floor, and can guard him solely as a catch and shoot guy.
2) His early hot shooting has teams keying on him more than at the beginning of the year when he was a bit of an unknown quantity. They are absolutely hounding him out there, which not only limits his good, clean looks, but also has to tire him out more because he's got to work so much harder for those looks.
3) He's starting to drift a lot more on his shots, which is probably a product of #2. He's working so hard to get open and trying to get the shot up so quickly that he's not taking the time to set himself and go straight up and down. He's coming off of screens or curls and his momentum is causing him to glide to the side, pushing his shot off.
4) You have to think fatigue really may be a factor. He's in great shape but this is the most he's played in several years. His legs really may be starting to get a little wobbly. Which is why I'd like to see Silent G get a few more minutes here and there, to keep him fresh.
5) He's ONLY shooting threes. This goes back to #1 kind of, because he's not driving. But seriously, no matter how good a shooter you are, you sometimes need to work from the inside out. When you go to the gym, do you automatically start launching threes? Or do you start with some form shooting to get a little bit of a rhythm going before you step outside and launch from deep? You need to get loose sometimes. And also, getting a couple easy buckets from inside is not only going to get you loose but will give you that confidence you're talking about.