I was going to say you need to strengthen your abs, but you're already doing decline situps. I had problems with these too and my physio pointed out I had lordosis (too much of an arch in the lower back). To correct that, he said to rotate the hips forward by contracting the glutes and the transverse abs, so instead of my ass sticking out, my wedding tackle stuck out. I'm not saying you have the same, but that cue helped me (as well as strengthening my abs).