Your core does a lot more than help you sit up, so here's a workout that will engage it in every other way.
A few sets of sit ups every now and again do not meet the training needs of your core.
Core strength and stability are built with movements like this, not trying to balance on a BOSU ball.
This exercise will light up the four muscles of your abdominal wall and improve hip-to-shoulder strength and stability.
Use this yoga flow to release the tension that might be the source of your back pain.