Breastfeeding tips?

I'm trying to exclusively breast feed my son, but its not going as planned and my pediatrician is making me feel horrible about it. Everytime I bring it up she says to just do it and not give him any other options and it'll work. It's very clearly not working.

I've had to supplement with formula as I am just not creating enough. I can tell when he's getting a lot and when he's not. I can tell that my breasts often take longer than they should between feedings to fill back up.

Pumping hasn't helped.

He will stay on the boob for hours and still not be satisfied when he comes off. He definitely doesn't have a tongue tie. I don't think its an issue with him. I just can't seem for my supply to come in no matter how much we increase feeding and eventually I have to give him formula because obviously I'm not going to starve him. I don't know. I really want this to work.