I would say just keep at it. The limits are pretty high with this thing (high downforce, lightweight, not exactly a huge amount of power), but you can still go overboard with it. That said, the easiest place to overtake is at corner entry, coz the AI as always tends to brake very early. Even better, they brake early while with the Jr you can really brake quite late and carry a lot of braking into the corner. I wouldn't take the AI's brake lights as any indication of where you should be braking, or even when you should be braking at all. One of the things I found frustrating about the Jr races was that the AI cars I'd be trying to get around would be braking into corners that you can take with just a bit of a lift or even flat out. But if you just pay attention to where the AI is being useless, you'll soon figure out where you can just plain drive around them or fire it down the inside. From memory Motegi is one of the tracks in the championship, and the S-bend near the start of the lap is a good example of it. You can with the right line get through there without touching the brakes (or even lifting completely off the throttle I think), whereas the AI annoyingly gets on the brakes. So even if you can't bash it down the inside for the initial right hander, you can get back on the throttle and go around the outside for the next left, and that sets you up for the inside of the next right too.
And from memory the suggested gear (if you're using them) often suggests you should be going slower than you really can in the Jr. So don't pay much attention to that either. You also want to slipstream where ever possible down the straights. You won't necessarily gain on the AI if they're doing the same, but as long as you keep up, you can take a crack at them in the braking and corner entry.
Really, I'd say most of the challenge with the RB Jr races (or GT6 in general I suppose) is just finding the limits of the car. Once you've sorted that out, it's basically just a time trial with some car-dodging along the way. The first race at Silverstone is a good introduction and a good one to do and repeat, as it has some good fast corners, and nice wide corners, to help show you just how silly you can be with the car under brakes and with corner speed while still getting away with it.
glad it helped.
