How do I make sit-ups harder?

I’m at a level of doing sit-ups (on a stability ball) for 2 minutes with 35lbs on my chest X 5. I use a dumbbell. My problems is this: I can’t add any more weight to the dumbbell without killing/bruising my clavicle as I do the sit ups.

I love the stability ball as it takes the pressure off my lower back where I have a vertebrae that is a tad arthritic (from an accident years ago).

Any ideas?