3 months nd 10lbs

Feeling so discouraged. My baby girl is 3 months nd only 10lbs!! Apparently its because of the hole in heart(which shes going to have surgery May or June) but i cant help but feel like its my fault..i EBF and i just feel maybe I could have done more...idk...now she has to start meds to get her weight up for the surgery and they may make her be on bottles so she can get more calories..im just heartbroken because shes the only baby ive been able to breastfeed with and now thats being taken away......i hate her heart condition!!