Flagship Credit Acceptance Corp. — Repossession of a vehicle

Address: 77590

I am trying to return a vehicle i had financed, but i encountered major health issues and i cannot afford it. I want to return it, a voluntary repo. but they want accept unless i sign paperwork stating i pay whatever amount is leftover from them trying to resell the vehicle. I refuse to sign such documents. So they will not pick the vehicle up. They want me to continue providing insurance for the vehicle. I do not know what ro do. What are legal obliagations, they are harrassing me at home and at my job.