I would have to agree with the others' suggestions, that the dominant breed showing in your girl is not Mastiff.
I see her as An American Staffy cross (I don't know why but her tail and elbows down looks kind of "heeler-y" to me.
Determining breeds visually is near impossible to get it accurate. Some dogs can have 8-10 different breeds in them. A DNA Test (Available by order online, give or take $100) might be your best bet if you really want to get close (and even then, isn't 100% accurate)
But I would stop calling her a Mastiff Cross and Start calling her an Amstaff cross.
In regards to her behaviour - In order to give you some suggestions on how to progress, as the above people have suggested, we really need some examples and scenarios. (Eg. some people might say "my dog growled at my toddler" but when you put the whole thing in perspective, you find out that the child had a biscuit and the doog took it and the child tried to get it back, but without the whole story, there really isn't anything that anyone can suggest.
Her breed won't have a lot to do with her behaviour. Environment, stimuli, previous experience and exposures and training techniques will make up the reason for most (if not all) of her behaviour.
Hoping we can get some more info in order to give some insight.
Asha - 22/05/15 Wolfhound cross a Dogue De Bordeaux X American Bulldog.