I'm sorry you're not enjoying your puppy - sorry for you, and sorry for her too.
You really have to WANT a dog. As you're discovering, they're a lot more effort-intensive than cats, and the level of effort doesn't necessarily decrease with the years. They always have to be walked and exercised and trained. They get sick and need (sometimes expensive) care. Your house will never be as clean as it was pre-dog. So the first thing to decide is whether or not this type of lifestyle is actually what you want. If it isn't, there's no shame in admitting it. Sometimes you try stuff in life and it doesn't work out.
For the walks: If your dog doesn't enjoy walks, there's something wrong. Are you sure the harness is fitted properly? Some harnesses restrict the dog's shoulder movement and can be painful. Have you tried walking her with a flat collar and leash only? Are you doing anything to make the walks interesting for her, or is it always a chore for you? Dogs are good at picking up on human emotions. If you're frustrated or exasperated at having to walk her, she may have picked up on this.
Potty training: She's just at the age where you should start to see some improvement. Most dogs, regardless of what you do or don't do, will become house-trained at around six months of age. Before that, accidents are inevitable. However, it's a bit unusual for a 5 month old dog to have daily accidents in the house. If that's what's happening, it doesn't sound like these are accidents, it sounds like she thinks this is what she should be doing. Are you sure you're cleaning the carpet properly, with a cleaning solution that removes all traces of the urine? If even the slightest odour (detectable by a dog) remains, the dog will continue to pee in the same place. If you have wall-to-wall carpeting that can't be removed temporarily, I would suggest getting it professionally cleaned. If it's a throw rug that can be removed, I'd suggest taking it away for the time being. I always remove all our rugs when we have a puppy in the house, and only put them back when I'm sure the accidents have finished. And for house-training, have you made sure she really understands that you want her to go outside? For example, have you consistently rewarded her when she pees/poops outside? Have you ever punished her for peeing indoors? If the answer to this last question is "yes", you may inadvertently have taught the dog not to pee when you are around. She may have understood that you don't like her to pee, period. So she might actually be holding it while you are with her outside, and releasing it indoors, when you're not with her.
If you're confident in the house-training that you've done, you might consider taking her to the vet and having her tested for a UTI. From your description, it doesn't sound as if this is the case, but it may be worth checking. I'd lean more towards her having learned not to pee when you're present.
For the cat: Is there a room that you can block with a baby gate, so the cat can go there but not the dog? This would give your cat a safe place to retreat to. Otherwise, have your pup wear a short leash in the house (only when you are present - never leave the leash on when you go out), and use it it break them up if you think it's getting out of hand. For information, we used to have a cat who loved to wrestle with our dogs. She would seek them out and torment them until they played with her.
For training: The fact of having passed one obedience course at 5 months of age means nothing at all. Just because she is trained to do certain things in class, or in your kitchen, doesn't mean she will do them in other contexts. Dogs don't generalize well. You have to practise often, in different places, for long periods, before you can claim that your dog is trained in a particular behaviour. And Golden retrievers are basically working dogs. They not only thrive on training, they actually need it to function well in a human household. I would say that a year of ongoing obedience classes and regular practice at home and elsewhere is a minimum to get a well-trained, pleasant companion. My suggestion would be to enroll her in another class and keep going until you're happy with what you have.
And most importantly: remember that your dog is still a puppy. Expecting her to behave like a fully trained adult is unfair to her and unrealistic on your part. A Golden will normally "settle down" into adult behaviour at two or three years of age if you're lucky. Until then, what you're going to have is a bundle of energy that needs training and attention. And even after that, she will need several daily walks, plus other activities. Owning a dog is indeed a lifestyle.
I wish you the best of luck with her, and hope things work out for both of you.
Ruby 13-01-2007 to 18-03-2015.
My dog of a lifetime. I'll miss you forever.