Firstly, it's not a case of "admitting" anything - not all dogs are straightforward, and it sounds like you haven't had a great deal of appropriate support from those you have paid to help you.
I think the training class is a great idea - it will give you and him something to focus on together. It's unbelievable to me that you were effectively thrown out of puppy classes - we have occassionally had pups come to puppy class who are so anxious that the trainers have offered to arrange some 1:1 support instead, but generally we work with the pup to get them to the point where they *can* participate in the class... to just send you away seems ridiculous.
I woul firstly recommend taking Archie for a full check at the vets - and specifically insist on a thyroid blood test - as it is always wise to rule out a physical cause for his behaviour difficulties.
I would also recommend finding a (better) behaviourist - perhaps if you post where you are people on here may have recommendations for you. I could easily understand you being reluctant to do this after your experience with the person you found, but guarding behaviour especially can be very complex and it is very difficult to advise on this over the internet.
Having said that, a few things could help in the meantime (and at the very least will do no harm).
As others have said, getting the balance right on exercise, brain-work and relaxation can really help. I'm not sure what exercise and training you do with Archie, but I would recommend several short training sessions a day (this can be just 5 mins here and there, and you can incorporate some training into walk time as well). At this point keep the training simple and stress-free... Likewise a couple of brain-games would be good - "find it" is a good one (and you can again do this on walks as well as in the house).
As far as learning to switch off, this may help =
http://dogscouts.org/Protocol_for_relaxation.html I would agree with the suggestions to try crate-training Archie if he is not already crate-trained - it may well help with the toiletting problems and will give him a safe place to retreat to. Introduce a crate very gradually, and make it a fun place to be... I can go into more detail on how to do this if you are interested.
Finally, I would recommend hand-feeding Archie for now - this can really help with guarding issues, and certainly improves the bond between dog and owner. And I know it is easier said than done if you are not sure what he is likely to guard, but try not to leave anything about that he might guard. If he has something he shouldn't, don't tell him he shouldn't have it, but tell him how clever he is and encourage him to "present" it to you. You may have to start with swapping for a really high value treat, but in time you can train a retrieve and encourage him to bring all kinds of stuff to you for a reward (which may be a treat, but also could be praise, a game etc).
Also agree with previous posters who have mentioned looking at diet, and that he is at the age where he is likely to be most challenging - Honey's guarding issues with other dogs appeared during her adolescence.
