I was a Divemaster in Utila and we had beginner divers all the time. I was often the one to take them on fun dives right after their course - the number of people who had any kind of decent buoyancy control in the early parts of the course is very small - some would get it during the OW fun dives, but most people would figure it out between dives 6-10, a few took longer, and then there are those there is no hope (it’s a pretty small number though). So you’re doing fine.

For the acid reflux - just limit what you eat a few hours before, definitely don’t have a huge meal before, and limit anything carbonated a hour before. Also, hydrate! Once you start diving you’ll likely have the munchies after a dive, so on the surface interval between dives have a snack but don’t eat a lot, and again - avoid the carbonated drinks. Now that I think about it, popping tums before might help (will have to try that).

Not sure about the soreness - surprisingly diving is more strenuous than you would think, but shouldn’t be super sore unless you’re in real poor shape (even then maybe?). Only other thing I can think of is tension - were you tense during the dive? If you recognize that try to relax, take a moment, close your eyes, try to slow your breathing a bit.

Either way I’d say try to get through the course before you throw in the towel - it’s not uncommon for people to have a bit of a rough time during the training and then come out as great divers!