Try doing it with your ribs closed, looking as if at your belly button.

When you do it against the bench, think of fixing your scapula to the pad and moving your body like a seesaw around that axis. Don't forcefully increase your range of motion while moving your hips.

When going down, don't lower it too much; just lower it to the point where your back doesn't bend. You also need to maintain intra-abdominal pressure while going down. If it still hurts, you should reduce the weight.