okay, lets not make excuses by looking at the bike. A '00 CR125 will turn with the best of them. The bike isn't the weak link here, its the rider (no offence, as the rider is almost always the weak link, especially when we are talking about such a great new bike).
you say you ride in tight single track, covered with leaves. Ill assume that there are trees close to the track. This can be really difficult, especially if the trees are barely a bar width apart, and if you are losing your confidence through here, then obviously the XR100s are going to be making ground on you. The only real tip here is ‘don’t look at the trees, or if you start to lose confidence, you will go where you look’.
Staying in the ‘attack’ position and using your legs to steer the bike is a good way of getting through slow, numerous turns. Its no good sitting down for the turn just so you can get up again for a second or two to get over a rough section. Using your legs will conserve your energy. Keep your elbows up for good control. But it doesn’t work everywhere, just under rough terrain with tree roots, rocks and so on. In these conditions, standing is a must, as it allows the bike to work under you. Another advantage of this position is the ability to use the rear brake to slide around tight hair pin type corners. Once the rear wheel is locked and swung to the correct direction, don’t dump the clutch, ease it out. Remember the ‘smooth is fast rule’.
You can also sit down for the turns if its wet and slippery. By staying well forward on the bike, you can just use your weight to tip the bike into the turns. Again, this will save energy which is important after 20 or so consecutive turns. Don’t extend your leg out like you normally would, because in tight single track, there is usually a tree there and you run the risk of nasty injury. This is why its important to sit well forward on the bike, as you don’t have the leg there to add extra weight to the front wheel.
Can you be a bit more specific about your problem? Its more than likely just a confidence thing, so practice makes perfect, but just like all aspects of bike riding, you need quality practice using the correct technique. Practicing without technique really is a waste of effort, but only if you strive to become better. Set up some cones or rocks in a line and practice the above method by weaving in and out of the cones. This way you don’t have to worry about hitting tress, and just concentrate on technique. Try sitting down and standing up, and see which one you prefer. Don’t forget about learning how to brake slide around turns, as this is quite a good skill to learn, if only to develop better bike control.