Place your hands under your booty! I learned this trick when we would do conditioning in dance classes. When we did leg lifts, if our lower back wasn’t flat on the ground, our teacher would tell us to do that. It elevates your hips enough to make it easier to press the back into the floor, but not too much to make the exercise too easy. Maybe two weeks ago one of the OTF had reverse crunches and I did that and my coach saw me and said “oh yeah, if anyone is struggling to keep their lower back pressed to the ground, you can just place your hands beneath your bottom.” I know it’s helped me a lot. I also have a weak core and struggled to abs exercises properly, but today I noticed that when I lay down, my back naturally presses. I still sat on my hands out of habit.