My begonia is dying. It has been 2-3 weeks since i bought them, it flowered but now some of the stems are wilting and it looks like this. the plant has become so bare now. it is in terracotta pot and the soil is well draining soil mix which can hold some amount of moisture

i am keeping the soil moist. i don't know what's wrong. btw it is my first time with begonias. pleas help me..i want to save it