Sorry but none of the advice you have been given so far is correct.
EDIT: Sorry, davemc is on the right track...
First of all: please dont ever hit your dog. Its cruel-they dont understand causality the way a child might (personally I'm not even for hitting in that context) so it really is just inflicting pain on them for no reason. Also your dog loves you in with that uniquely canine sincerity and it hurts their emotions even more than their body since they cannot understand what you are doing. I know people whose dogs flinch when the owners reach behind an ear to give them a scratch, its heartbreaking.
You need to understand how a dog thinks - to a certain degree your smacking and yelling is attention... good or bad-sometimes it doesnt matter. Remember that each dog has an individual personality, some individuals dont discriminate.
Become familiar with the concept of positive reinforcement. Dont do anything when your dog 'goes' in the house but definitely put her out once you find it. What you want to do is catch her in the act of 'going' outside then praise her like crazy! Really go nuts, full babydoggy talk, high pitched squeaky 'good girls'...even get your body language involved, ever seen puppies play? They tend to taunt one another playfully by pretending to run or jump but ending up in a 'false start' sort of posture (legs out front; head back and down; hind legs bent) - that body language is loud and clear approval and love for dogs, they all know it as a way to initiate rough play which is integral to pack hierarchy... never let her win a game once shes in this mode, ever. But use it as a non-edible treat, i.e.: she must earn this kind of play by doing something good.
Another obvious problem you have is that she does not respect the 'den', I bet (based on your description that she is allowed all over the house at any time, possibly even on funiture. This is the worst thing you can do. How can she 'park one' in the computer room for instance without you knowing if she dosnt have free reign of the house? If you arent in a certain room- she has no business there.
Imagine your home as a cave, subdivide it into three sections starting at the back gardens back wall and ending up right at the front gate. Good dogs are free to roam the the back and middle areas. Very good dogs get to go out front and are even taken for walks (beyond the front gate IOW). Bad dogs are not allowed out of the first area and maust remain in the back yard - it may take some physical force and a bit of patience on your side but she must go "OUTSIDE!" (loud, assertive voice) on command, leave the back door open but do not allow her in the house. You must assert yourself until she will sit right there, dying to come in through the open door but will not dare until you say "COME". Keep at it until you can actually leave her there for about 5-10 minutes without her trying to enter the house. Always return at some point and tell her to "COME", she should never enter under any other conditions after being told: "OUTSIDE!"
At this stage you have a very effective (non physical) form of discipline: "OUTSIDE!"
Dogs hate it, they are separated from the pack, lonely and most importantly: relegated to that cursed 1st 3rd of the den (lowest rung on the pack hierarchy.)
You mentioned her sensitivity - I am willing to bet that your tone of voice effects her profoundly whats described above will most likely be very effective.
You will find that you can spot her about to do something bad and just say in a very circumspect tone: "OUTSIIIIDE?" going up at the end as if it were a question and she will probably respond by changing course.
Another important point is that she identifies her bed (you most likely have some sort of doggy bed for indoor use as welll as the outside dog house) as her 'correct' spot to sit when she is allowed inside the den but is not interacting with other members of the pack. She should retutn to it on command ("LIE DOWN" or "ON YOUR BED") and she should never be allowed on furniture-she has a doggy bed. Our dog can be told when he is misbehaving: "Do you want to go outsdiiiide or on your bed?" 9 times out of 10 he will immediately return to his position on his doggy bed usually facing the person who scolded him in an attempt to earn apporval "Look I'm on my bed dont put me outside!"
Once you have these things down and the culture is such that this is how it works, no other way and no deviation from this formula ever I think youll find your dog housebroken (hate that word BTW) - it makes no sense for a dog to soil the den.. they just dont operate like that and thats why once you tap into that psychology things like this fall into place on their own.
You may get the odd mishap I'll come downstairs once every few months to find a number one in the corner near the back door but I'll also remember a those times a 3am wet-nose in my armpit that I ignored
Let me know how it goes and if you'd like any more info... a year seems a long time to deal with a dog soiling the house, especially when there is no need
is to close down her environment so that she cannot sneak around where you cannot see her.
I don't have the heart to keep putting her nose in her own poo and yelling at her when it clearly isn't working. You don't think I would have come on here if I hadn't already tried the basics? We've put her in the closet, we've used a stick on her behind - not too hard because she's fragile, but she definitely feels it.
) but its not going to help to hit becuase it doesnt tie back to that initial action along a linear timeline the way humans understand it.