Like Bigcat, Kitah and Amelia said before, your pooch has inherited the traits of her breeds, just accept it and love her for what she is. Boxers are a playful breed, I bred and showed them for years before I got into breeding and showing staffys, and got immense enjoyment from them. They respond well to training using positive reinforcement and one of mine went on to be an national champion in obedience and agility, so just have fun with her! Keep an eye on her eyes though as boxers can be prone to entropic eye disease where eyelids need surgery. I've never owned a mastiff breed but know they can suffer from heart disease and hip dysplasia due to their size but with exercise and a balanced, lower fat diet she'll be fine. Keep her on the slim side with plenty of free running exercise (just limit hard exercise while she is younger to avoid damaging developing bones and joints), keep her well socialised (they make excellent watch dogs) and you'll be fine!!!