"You're riding with a hollow in your lower back, which pushes your pelvis and seat bones to the back of the saddle and out from underneath you. This makes it hard to have a truly deep or following seat and robs you of some security."
The good news is that the rest of your position is rock solid, and this is not that difficult to fix. Once you feel what it's like to have a relaxed back and seat bones underneath you; you'll be apply to reproduce with it very little effort."
Now how do I fix it? I really need help.