Dog is 6 month old mastiff who we have had since 5 weeks old (lived here with mother and other pups). Cat was very unhappy and wouldn't come in the house for months, then progressed to only staying in ds's room. Dog has chased her upstairs a couple of times when we haven't been able to get there quick enough, but he has never hurt her. He used to bark and do play bows at her but we have managed to stop him doing that mostly.
Now cat has progressed to lying on the back of the sofa in the front room, even sleeping there overnight while dog sleeps on the same sofa! However if he tries to sniff her (which he does often) she hisses and tries to bop him on the nose. Today she hissed at him because he yawned while sitting next to her
He looks so sad when she does this and whines 
, he just wants to be her friend so much and sniff her! He sits as close to her as she will allow and keeps looking at her out the corner of his eye, because if he turns to face her she hisses or yowls.
Is there anything I can do to help the situation? Or is it just a matter of patience and time? I would love cat to realise that he won't hurt her and is just being friendly. I dream of a day when they might curl up together and be affectionate.