When you make rasogolla it's important to retain the softness of cottage cheese. When you make cottage cheese turn off the gas the moment the cheese seperates itself and can see the water . Strain it ideally for 30 mins in a muslin cloth not more or else they crumble while making the balls. Knead the cheese well. Kneading and softness of the cheese is the key to make perfect rasogolla. Hope this tip helps.