Hindsight is 20/20 of course, but I would have filed for the I-751 with a divorce waiver to begin with. People get divorced, it happens, and as long as you were able to provide evidence that this marriage was entered in good faith, this would not have been a huge issue.
It looks like you were advised to go the abuse/extreme cruelty way, but I think that just made it way more difficult. I am not sure of your chances with changing it to a ‘normal’ I-751 with a divorce waiver, hopefully other members have a better idea about that.