My 5 month old doesn't sleep during the day!?

My 5 month old baby girl doesn't sleep for longer than 45mins at a time during the day, and I find myself putting her to bed up to 4 times a day. She has been doing this since she was 2 months old, and it is really becoming exhausting. I try to resettle but it is emotionally draining to have her cry and cry as I do so. She sleeps fine at night, and she puts herself to sleep after her evening milk, but during the day it just doesn't happen. Does anyone have any tips? Will her sleeping pattern get better....