difficult eater

Kitty
My baby girl will be 10 months old next week, and she has been really difficult to be fed since 5 months old. Now I give her a variety of food (purée, porridge, pasta, baby cereal) three times a day. She has to be entertained by toys/objects in order to eat.
During her 9-month checkup, the doctor told us to keep her food journal because she's underweight. Since she doesn't take bottle, so the doctor's advice is to add formula powder to her meal. 
Each spoon-feed is at least an hour long, and she doesn't eat much. She doesn't have favourite food so far, but she enjoys finger food until she gets tired of picking up food and chewing. 
Please share with me if you have or had a difficult eater like my LO and how you dealt or would deal with your baby. 
Thank you from a mom who desperately needs your help!!