When a girl come into the life of a son then mothers start feeling insecurity. They may feel that now her boy start giving importance to his wife.May be this is the case. But if you massage the ego of your mother in law then may be she can change her attitude. If this is true what ever you explained about your husband then you should also be act smartly.
If you argue your mil, try to prove her wrong then I can assure you that all your efforts will go invain. So make sure that you should never argue with her. Try to praise her. When your husband will see your efforts then his attitude will change. Forget about past and do a fresh start.