One of the most experienced instructors in gliding (Derek Piggott) has several theories about negative-g sensitivity, having seen it in a proportion of his students. It might be mentioned in some of his books.
When you say: "the one thing I need to do is sort out my negative G reactions", do you mean you find -ve g unpleasant or that you react in an uncontrollable manner to low/-ve g by doing things with the controls that don't help the situation, e.g. pushing forward?
If you're experiencing the latter, go to a senior instructor/CFI/coach immediately and explain your problem before flying again. If you get reduced-g near to the ground and have an involuntary reaction to it, things could go badly for you, your P1 and the glider.