Expending too much energy breastfeeding?

My doctor told me today to stop breastfeeding to feed my LO. We were combi feeding prior to this, as in breast for 10 minutes then top up with formula, due to low supply.

LO is now 12 weeks old this week, and has been having weight gain issues. One week she gains, then for the next couple she stays the same with little to no change in her feedings.

Today the doctor said she is "expending too much energy" breastfeeding / using me as a pacifier and that's why she isn't gaining. She does use me as a pacifier to fall asleep a large majority of the time, and refuses an actual pacifier.

I guess my question is, would she not be expending energy on a pacifier? or crying when she screams when she gets tired and wants my boob when shes tired?!

I'm so depressed and sad over this and had come to terms with no exclusively breastfeeding but being told that IM the reason shes not gaining weight because I keep giving her my boob when she cries has me all sorts of fucked up today.

Anyone have any experience with this? or thoughts? I guess I just never thought that her being on the boob would be expending THAT much energy and thought the balance of energy expended on boob vs crying, boob would be the better choice but maybe I am wrong