Acting like she's starving!

My 3week old girl is always acting like she is starving, even after she is finished eating. I'm not producing enough breast milk, so I've been using 3-4oz similac supplement a couple times a day in between breast milk feedings. Well she was still acting like she was hungry so we tried similac advanced, but I think it's hurting her tummy cuz she has been more fussy and gassy with that. I don't know what to do and I feel like I'm a horrible mom for not being able to feed her enough breast milk.. What do I do?? Is there a different supplement that will satisfy her better?