Good morning, I am sure you are trying diet with good exercises at Gym, Whenever going to gym take some protein shake or 2 handful dry nuts to save your lean muscle and reduce fat . During workout take sips of plain water or lime water, afterwards take healthy balanced high protein low carb diet with proper hydration.

I understand your concern. For reducing fat n increasing muscle mass, you need to have a proper exercise schedule with a combination of cardio n strength training. Along with that you need to take a good diet, so that you don't feel deficit of energy while you work out. That energy should come from wholesome cereals n pulses and good fats like nuts n seeds. If you are complete vegetarian, you can combine supplements of whey n caesin both for fulfilling your protein intake.