Regret adopted dog one year later

My fiance and I adopted a one year old male dog from the shelter in December of 2019. They told us he was a shy but sweet dog. The third day we had him he severely bit my fiance's hand with no warning. Here we are over a year later and the dog still growls, barks and just plain doesn't like my fiance. I'm terrified that the dog will bite him again. The dog is so nervous and anxious he destroyed a door handle chewing on it. I feel awful but I don't like him. I don't want the anxiety that the dog gives me every day. The thing holding me back from retuning him is the guilt I feel. That he won't understand why. But the dog is so anxious around anyone but me it creates my own. I just feel drained and over trying to build a relationship with this animal 😭😭 my fiance supports whatever I think is best. At this point I feel like it's affecting my mental health. At what point to I have to put aside from the guilt I feel over failing this dog and worry about myself and my health?