OK - a few things to unpick first:
What breed is the dog? This is the most important thing. Most breeds have inherent traits which will affect the best way to train/motivate them.
How old is the puppy? Again, depending on breed, age matters.
How many are in your family? Am guessing 2 adults and x children. The age of your children will have an impact on how to manage your puppy.
What is your home set-up? Are you rural? Are you in the suburbs/a town/a flat? Do you have a secure garden? Etc
The “puppy blues” are a real thing. Pretty much everyone I’ve interacted with over time has had that “what the fuck have I done” feeling at some point during the puppy stage. So DO NOT beat yourself up for feeling like this. What you need to do is get practical. Having answers to my questions above would help, but in the absence of info specific to your set-up, some basic advice would be as follows:
For the food possessiveness. When dog is given its food, train it to sit before bowl is put down and wait until you give the signal for it to eat. Manners are really important! If you have young kids, do not let them near the dog when eating. Do not let anyone in the family feed scraps from the dining table etc. if your dog is in the room with you when you are all eating, do not let him beg/pester at the table - teach them early to move away and lie down elsewhere. If you do want to give them some scraps, it should be once everyone has finished and those scraps go in their bowl along with their usual dinner.
With regards to training, the best thing you can do as a family is to agree on your commands. The absolute basics are “sit” “down” “stay” plus “wait”. If possible, look into videos demonstrating hand signals for those four commands. Try to develop those if possible. Every voice is different - on top of that, every voice in a different emotional state is different. But hand signals remain constant, no matter how frustrated you are. Also, I see/hear too many people yelling “sit down” to their dogs - poor dogs are likely thinking do you want me to sit or lay down?!
Researching the breed of your puppy will be a big help. You need to know what they were bred for - that will give you guidance as to what their strengths are. There is no point trying to get a terrier to retrieve something with a soft mouth! Different breeds will have different motivators - you need to know yours to give you a starting point as to how best to channel their training.
Also, look into recommended puppy schools in your area. They can be a great way to learn some of the basic training methods whilst getting your pup to interact with other dogs in a controlled environment. And also a good way for new owners to vent with each other!
I had a giant breed who posed a few challenges as she matured. The best advice I got was to be consistent with your training/commands. It can be a PITA to begin with, but it’s worth it. My girl sat automatically for her food, she sat and waited for me to step out the door before she followed. She knew not to jump up at people - she’d get fuss if all four paws were on the floor. If you’re consistent with your basic expectations, they will become second nature for your dog.
As an aside, it’s a bit disappointing the amount of “rehome him” posts. Its a puppy - the owner hadn’t had time yet to get over the shock of how much of a responsibility it is. As I said earlier, most people I know who’ve had a puppy have had that “wtf” feeling for a while. None of us told each other to rehome said puppies.