I've started calisthenics about a month back. Prior to that I've been lifting weights at the gym for a decade. I feel my abs only while doing a plank. During crunches, I hardly ever feel abs - it mostly comes on my upper back or spine or neck. I can hardly do sit-ups since my lower back hurts after about 5 reps.

I seem to have a curvature in my lower back that prohibits me from doing hanging knee raises or hanging L-sits also - everything just hits my lower back and not my core. Even while doing dead hangs, I am unable to engage my core or do a front pelvic tilt - lower back remains arched.

What can I do to improve this?