Rant! Feeding baby

So my LO is 3 weeks old and he hasn't been gaining the weight he should be so my pedi recommended not letting sleep longer than 3 hrs straight and waking him up and feeding him. My MIL has been telling me not feed him on a schedule, whenever he wants to eat I should feed him. Fair but not when you have a sleepy head on your hands, who would rather sleep all day than eat. All of the sudden I get a message from my hubbys aunt (she's a lactation sp and helped me in the beginning)  telling me the same thing as MIL; which I am sure is my MIL doing bc the message was out of the blue
All I am trying to do is to feed my baby and when I see that he's not gaining the weight he should be, it breaks my heart. I am not starving him (or maybe I am, idk how my milk supply is doing) Would you be annoyed? I am doing my best and I feel like a failure and I don't need to feel like this. Am I just hormonal or would you be upset too? What would you do in my situation?