Ab workouts such as sit ups and planks hurt my back and neck, so what exercises can i use?

Core Exercises: Typically core strengthening exercises are the best to help relieve pressure and forces in the spine. I would try and google some or look them up on youtube to see descriptions. If you can do them yourself, wonderful otherwise ask your doctor to get a referral to a good physical therapist for further assistance. If you hurt that much, recommend getting imaging studies to see whats up.