Looking for some advice/help!

I took my two month old in for her check up and shots. Her doctor is concerned about her weight gain and suggested I supplement the end of feedings with formula.

I have to use a nipple shield because I have flat nipples. She is able to latch on her own now but I’ve only tried it twice because I don’t know what a good latch feels like and it makes me nervous that she isn’t getting milk with out the shield.

I feel very inadequate knowing she isn’t gaining a good amount of weight. She weighed 7lbs 11 oz at birth and 9 lbs 14 oz at her check up.

I feed her 8 times a day, every 2.5-3 hours. She takes good naps, pees and poops and is happy.

She eats for at least 40 mins (not all 40 mins is good sucking) and I go back and forth between boobs during feedings.

Any advice on how to fatten her up? And nipple shields?!