If you are rolling forward into a hunt seat position, it sounds like you are nervous and tense. Also, you may be making your youngster have a more difficult time loping as you are putting more weight the horses front end. Really think about sitting back, and pulling the core of your body backwards. Make sure you don't arch your back, you want it flat. Also, sit more on the backside of your tail bones. Not rolled under (that will put your legs too far in front of you). Your hips can move a little bit with the motion of the horse; that sometime works better than fighting against it. Look up (looking down makes you tip forward). Also, make sure your stirrups aren't too long - that can throw your body all out of joint.
Let me know if you have any questions about what I was trying to say (I can see it in my head, but can't explain it very well
). Oh, and if you have any recent videos of you loping that may make it easier to help.