The first pit(we think she is a pit) we got last year(one in the avitar) did the same thing.
It took me for ever to bring her around.Ok about two weeks, we talked to her gave her treats she just kept growling! then she attacked my chocolate I thought my husband was going to have her put down. But I talked him out of it,to make a long storey short, it has been about eight months and constsant treats and praise and ignoring her and she has improved so much. I can now hold her play with her my kids lay all over her, it's been a long haul for all of us but Isis has turned out to be awesome.( Isis was badly beaten as a pup, she just needed to trust humans again. Since then we have gotten two more baby pits that where going to be put down!
The lady we got them from was affraid of all this bsl.Isis is still a little jealous of the chocolate but they all live together and the chocolate is the queen.
good luck with your dog hope it works out for you.