What worked for me (and totally got me out of my horribly defensive jumping position) was "The grotty public loo analogy" on of my instructors told me this and it is absolutely perfect, and because of the eww factor it sticks in your mind too! And you can practice it!
Don't think about folding forwards, or standing up, or reaching for their ears as there is a significant risk that you end up pivoting from the knee and tipping making yourself insecure. Think about being desparate for a wee and having to go in a really nasty public loo. So you don't want to sit on the seat, but you need to hover over the pan. That is the position you need to adopt.
Try it at home in the loo and feel where you need to be. Then next time you are riding hoik your stirrups up (you may be struggling because you aren't shortening your stirrups enough, I was taught up two for jumping, but actually I need to go up about 5) and try the "grotty public loo position", keep that vision in your head and keep practising.
I find an handful of mane half way up the neck is better than a neckstrap too