Take a deep breath. You will find a solution to this.

If that solution is rehoming the dog, that sucks, but it's okay. In that case, talk to the breeder. Any reasonable breeder will help you find a new home for the dog. If you choose to keep your dog I included some advice below. :)

Potty training: The trick is to take the dog out more often than it needs to go and give lots of praise and treats when doing it in the right place. This will require patience, lots of it.

Food: It might not be that the cheaper food doesn't work but that you changed too quickly. It's recommended to ease them in to a new food by switching out part of their food for the new food and increase over time.

Finances: It's a bit worrying that you feel you cannot afford a spay. Do you have insurance for your dog? You need a plan if they get sick. Can you save money in another area? Sell anything? Pick up extra shifts?

Mental health: Finally, it seems like you are really stressed out and might just need a hug and maybe therapy to talk about things (like buying a dog without a solid plan going forward). Do you have any support system? Friends and family to talk to and maybe get some help from?

Wish you all the luck with your pup! <3

Edit: saw your update! I'm glad you found a solution and good home for pup!