i have to agree with a few of posting predecessors; clearly a paddling issue. you're not generating enough speed and getting hung up at the top. with the longboard you can get on a wave with a few strokes; a boogieboard can drop in late on almost anything without getting pitched; a shortboard needs a few more grunts to travel faster than the wave itself because when you pop up you slow down. paddle as hard as you can then paddle harder. you'll start getting them.
speed of paddling also keeps the nose higher. TAil slightly higher than nose, but weight up by arching your back, that advice is good. It sounds to me like you are in late and getting pitched over cuz the wave is already too critical.
I've never had luck shifting my weight fore/aft when paddling into a wave. I suggest... find the sweet spot on your board where you paddle from, so your board paddles most efficiently. Shifting your weight in either direction away from that spot increases drag and reduces paddling efficiency. As you're finding out, timing is more critical on a shortboard than longboard. Practice will improve that. You're board won't catch a wave until it's speed matches the wave's speed. Paddling HARD the last few strokes helps that happen. Then, popping up... it has to be quick and smooth, and it comes from your core. Use your abs to pull your body into a crunch, with both feet hitting the deck at the same time. If your front foot gets planted first, then your back foot, your pop up is too slow, and your probably dragging your back knee, too. Some people explode up from a prone pushup position, some people arch their back, straighten their arms, then pop up from there. Either way BOTH FEET at the same time! Then you shift your weight (front foot/back foot, toe/heel), depending on where you're at at that moment.