It’s hard to say without a physical assessment, but 42 kilos is very slight for a woman of almost any height! While it’s absolutely wonderful that you are a fitness fanatic, I would first have a body stat assessment done to measure just how much lean muscle you have, as you want to make sure that you’re not eating it away from over-exercising. A dietician or personal trainer can help you with this. Once you’re sure that you’ve got the calories in versus calories out ratio correct then go ahead with Crossfit and Sweat1000! I practise both, with great results. Remember, though, that these are general calorie-burning workouts, they aren’t tailored to your specific needs. If you need to build muscle, you will need to check that your weight sets are heavy enough, where your muscles are almost exhausted on the last few reps of every third set of each exercise. Also make sure you supplement your workouts correctly before and after you train to make sure your protein and carb requirements are being met.