Sandy, since you and I are in the same area, let me know what you find out about the horse place. I would love to tag along.
Since you say our dogs are so much alike, I wonder...did you have a problem with him biting? He bites all the time and most of the time he responds to me telling him no but sometimes he gets in these moods and won't stop. That happened today. My hands all torn up because he wouldn't stop and when I corrected him he just got mad and came back at me even more so I escalated my correction, he came back even more until at one point I finally had to take him down and hold his neck (heaven help me if this was the wrong thing to do) and even so he fought like a banchee to release my grib from him. I did of course release and then just sat there ignoring him for about 5 minutes with the leash only out about a foot so that he couldn't get away from me. Finally he sad down and relaxed. I said good boy gave him a pat and then got up and walked us into the house where I came upstairs and began typing this. He is lying down sleeping but not at my feet like he usually does. It's almost like he has to keep an eye on me from a distance. I read Ed articles on correcting and I'm hoping that my pup is not a soft dog. If so, I might have just ruined everything. Wah!!! Dang it, this whole thing seems to be getting so complicated. Still, I can't let him think that he can do whatever the heck he wants. Maybe I am just overthinking all of it. I don't know but let me know what you think. I can certainly relate to the "what if he" mode of thinking. I do it constantly it seems. I'm such a worry wort.
Not only are our dogs twins, but we may be as well LOL. Shoot, did I have problems with my pup biting...YES, exactly as you described it. Correct him and he comes back with more intensity, like he was mad. But I now believe it was more of a high prey drive. He perceived it to be rough-housing play (as Steve mentioned) and they love that stuff, actually can get into snarling later, but I believe it's still prey drive. The more the better, for them. But NOT for you.
Then I believe it develops into a bossy puppy later, somewhat dominating, because we, being badly hurt and cut up, begin to back up, get apprehensive of the pup, get even downright scared of the pain and other emotional reactions (I sometimes yelled in pain), off which I think some of these high drive dogs thrive. Nothing I did seemed to work (corrections, diversion, exercise, etc) and I believe the reason for that was simply put, my growing apprehension of him and apprehension of getting bit. I tried all the methods, but I still wasn't the boss. I forgot (literally) how to be calm inside and how to be the calm "boss", and I believe they thrive off that too, as they can sense it easily. My frustration grew and I wasn't even aware of what that was doing to me, and to my pup. The biting got out of control. But I still didn't know what to do, even resorted to a prong at age 4 months but he was wise to it and behaved wonderfully when it was on, but bit the heck out of me when putting it on and taking it off. I know he wasn't stupid (even though I thought it sometimes when he bit me), cause he learned EVERYTHING quickly. He got to the point where he didn't like me telling him what "not" to do. He would get mad and start biting me, actually coming at me. So what else could it have been but a pup starting to push his owner around. It seemed to start with a natural prey drive and progress to domination over me. He had me.
It honestly wasn't until around Christmas that I realized it was my fault (I think it's always our fault). He was a dog unlike any other dog I've had and he had me losing a few marbles. He did the basic sit before eating, I went out the door first, etc type of thing BUT these were all things that got him something he wanted (food, going outside, etc). It didn't amount to a hill of beans as far as leadership in his mind was concerned.
I hate to make this a book, but there were no easy answers in my case. Around Christmas it dawned on me that the leader I had been to all my other dogs wasn't at play any more. My leadership had never been challenged before and it took me by surprise, so much so that I stopped being what I used to be with my other dogs, calm and confident. So I set up some groundrules of my own, most of which was actually for me, to keep me solid. I stopped playing with the pup, except to throw things for him to get. I stopped petting him (which would get him too excited), stopped doing whatever made him too excited. He needed calming and I needed confidence. I stopped talking all sweet to him, and became rather business-like. I only met his basic needs. A lot was based on some tips I got from someone on this forum. A lot was also based on Ed's article about setting the groundwork with an adult dog, but I figured at that point it was all about making me behave differently and his article applied. It worked - I got stronger and more confident.
So my behavior and interaction changed with my pup, and he is much more settled down, and has only resorted to trying to bite me once since. I expect as much from him as I did from my other dogs now. I have by no means won the war, but I'm winning battles. The more I win, the better he gets. He is responding and being my buddy, although I know he's not done challenging. That's OK, I'm up to it now.
A couple of more things: you didn't ruin anything, your pup just got taken aback by you holding him down. By the way, that probably won't work, I can almost bet you. He'll figure out what's going on and get "mad", defensive, whatever you want to call it, if he's anything like mine (and he seems to be). Then he'll start fighting you with more biting, you'll back off, or hold him down again, and the cycle continues. For him it will be a littermate's rough play for domination.
Also, there's nothing wrong with a soft puppy. I rather doubt your puppy is soft. If our pups were soft, they would take a correction more seriously. It wouldn't take much for them to see who's boss, like my other dogs did, and probably like your Corgis. I had a GSD who when told "no" would never EVER do whatever it was again. I was spoiled.
I'll get in touch with that gal I met from the horse rescue place and then PM you. We could meet there, that would be fun