Help please new baby with heart condition

Baby girl was born 7lbs 5 oz .. she went down to 6lb 12 oz .. she's at 7lb even for 6 days now. Doctor isn't haply with her lack of gaining weight and wants me to do formula after each feeding. Should I just try to power feed first before introducing formula. She said with her condition the not gaining weight could he an issue. Any advice?

She has a Right aortic arch with aberrant left subclavian artery and a small vsd.

I've been letting her sleep for 4 hrs because she sleeps so good but I have to wake her up every 2 hrs now to feed but I don't wanna give up nursing .. my supply is good it's not a supply issue and I feel like she's eating alot. She guzzles it