Andreia
Berlin as in Germany? Your climate zone is 8a and the Fortunella Margarita/Citrus japonica should be hardy in your climate.
Is the plant indoors or outside? The photo makes it look as if it's growing indoors near a heat duct. Perhaps being to near the heat is causing the plant to become dehydrated? Maybe move it away from the flow of hot air and increase the humidity a bit. That's all I know. I'm sure someone will be along with better information soon.
