@erica
i had the same issue, and i was recruiting my back muscles way too much to compensate. i'd have such a sore lower back after core work ))):
main things that helped:
> stomach vacuums
> hollow body holds (make sure your back stays FLAT)
> leg raises (also back flat!)
i've found it also really helps to do lower reps and really focus on form + activation. like for leg raises, i don't try to get my legs 1" off the floor or whatever, i lower them down until i can sense my back wants to start curving off the floor and then i stop and hold it there and try to focus on using *just* my core to hold my legs up
also if you can use your hands to touch your muscles and create that mind-body connection