What type of cardio does your trainer have you do aside from boxing? Even if you are into boxing, it seems like you should be doing other types of cardio exercises; biking, running, swimming, jumping jacks, jump roping, etc. You get various endurance trainings by doing different types of cardio. Rocky Balboa was always doing various types of cardio

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Also, what is your diet like? To build muscle and lose fat it is best to eat 5-6 meals a day in small portions and eat lots of protein.
Hang in there!
Brianna
witnessforfitness