Have you tried keeping a sponge between your lower leg and the horse? It will help keep you focused and will give you a visual cue for when you go wrong.
You strap it back mate. The it won't move. ;D
I do a lot of two point position while trotting and walking down long sides. This helps you strengthen your balance. Also putting your hips back and chest up helps. Don’t lean forward and fall onto your horses neck. Grabbing mane every time you jump also causes you to go into a position that causes your legs to slide back