I can understand your concern – your kelpie x sounds very unwell. Regurgitation at such a young age along with weight loss and weakness require thorough investigation for a congenital problem with her throat oesophagous, stomach or nervous system.
I would recommend seeing an internal medicine specialist in your area (either ask your vet who they recommend or search the internet for veterinary internal medicine specialists). They are very experienced in dealing with unusual or complex cases and have highly advanced knowledge.
Best of luck with your little girl.