Robin, congrats on doing all those sit-ups!
You're going to have abs of steel when you reach goal!
OK, the best way to protect your back is not to do traditional situps at all. Do crunches instead:
Lie on the floor with your knees bent and back flat against the ground. Cross your arms over your chest (this makes your abs do all the work - when you put your hands behind your head, you're hauling your neck up with your arms, not your abs). Crunch up s-l-o-w-l-y while exhaling and sucking your belly button into your spine. Really squeeze your abs. Hold and come down s-l-o-w-l-y. Try going up for a count of 6, holding for 2, and down for 6. It's a small and controlled movement, but if done correctly by contracting your abs muscles, you can really feel it.
This video shows the correct range of motion, but please ignore her having her hands behind her head! Crunches
The way to progress crunches after they become easy is to change your leg position. Try knees up - same technique - and then legs straight up in the air, ankles crossed.
The Washington Post (the link I posted above) has lots of other videos for different ab exercises that you can do at home. Core Exercises
Just click on the name of the exercise you want to see.