Remember, your "core" isn't just your "crunch muscles." It's what locks your pelvis and your shoulders together.
Here are some ideas.
If you want to get better at the ab wheel, find something kind of like it, but a little easier. Something you can get some work in on
-Ab Fall outs on blast straps/TRX, with the handles kinda high
-Stir the Pot, with your elbows on a stability ball
-Ab wheel on your knees
Then build that up for awhile. Once you double your work load on the "easy" exercise, and your mid section is improved, go back to the ab wheel.