I've never come across a collie that doesn't like to train or work, certainly never come across one that looks fed up staring into a corner :-O Well, not unless there's a good reason for it!! But collies are all different & obviously what makes them want to work varies too.
Collies are very, very bright, as you know. They know if you are trying to 'gee' them up but if the emotion isn't there from you they also know that too.
I've got a girly who the instant I used to try to gee her up with an excitable voice, etc, she just looked at me and walked away. She knew I was being false (I don't use an excitable voice in every day life with my dogs). The trainers I started out with had me talking quite loudly to my dog (I never understand this concept of talking loudly - in an excitable voice - to your dog that is right next to you and has the most amazing hearing) & in a sing songy tone (so unatural for me), becuase I found it a strain, my girly just switched off. She must have wondered what on earth I was up to. We both ended up hating our training. We ditched our trainers, took a complete break and returned to training with someone who worked with me, allowing me to use my voice in my manner, ie, very quietly talking to my dogs. My collies respond to quite, yes they'll respond to excitable too, but that's not when they give me the best work 'cause they're then excitable and not concentrating! But I can gee up my dogs in a quiet manner, and I don't have to sound like a loony on drugs for them to work for me!
Think about what she likes ... be it food, toys, or doing. Make a list. Prioritise what she likes - for my girly a ball has highest priority over a bit of sausage, but a leaf with a very quiet "swish" sound from me as I pick it up off the floor is the highest motivator of all :-D
If she'll play tug then that may be the thing that you need to make a really high priority item. Her absolute reward. There are ways you can increase the value of tug - the main one keeping the tug toy as your special toy that she's allowed to play with when you let her. There's a good dvd by Jo Hill, Motiviation Movie. Basically about motivation - includes a good section on how to get dogs who don't necessarily have a high toy/play drive to start with to work for a tug toy.
Also a good article on Susan Garrett's website about how to make a motivational toy
http://www.clickerdogs.com/createamotivatingtoy.htmYou can ignore the 'goofy acting' she refers to if it's more difficult for you, but the rest of the article about how to make a toy highly motivational is achievable for all.
If it were me, I would ditch formal training for now and start working on trying to get her interested & motivated, specially on a tug toy as she's partly there. If she's switched off in the class environment, you're not going to switch her back on in that environment, better to work on that at home and then take it beyond the home. Once she's playing tug, let her 'win' it occasionally to build up her confidence. If you can work with 2 identical toys that's better as once she's 'won' the toy you can show her the other one & regain her interest without the formality of asking her to give up her toy.
You can add some training into it once she's keen to play, but I'd only ask her to do things that you know she can and will do, a simple sit - then reward with the game (once she's onto the toy, or food, whichever you make the high value item).
We all have different ideas about how to train our dogs and it's a case of you working out what suits you, and especially what suits you and your particular dog situation - with a number of dogs you know that what works for one doesn't necessarily work for another. Some will back off at the slightest bit of pressure - and it might not seem like pressure to you. I was told a few years ago, smile when you start your training session - if the smile isn't genuine then postpone the training to when it is :-D She'll know when you're faking it!!
Good luck whatever you decide to try with her, but whatever you try you may need to stick at it for some considerable time, months.
Just to add, when I ditched my original trainer with the girly I mention above, we started self-teaching (obedience training I'm talking about) & used a clicker. She absolutely loved this. As lilyowen mentions, might be worth considering if you don't already use a clicker with her - if you don't clicker train you need to get your head around the concept before starting with her though.
Ooops, sorry. Just re-read your post - it sounds as though you're getting a bit stressed by the situation, this will most definitely come across to her, you wont hide it. You haven't failed in the slightest, you may not just have found the right solution for Gwen yet. There will be one, but it may just take you a while to figure it out. If she genuinely doesn't have the drive to want to train/work then that's something you will have to accept, but I'd stick with it for now, specially if she's very sensitive. The best thing may just to be completely remove all pressure from her for a few months and do nowt that she doesn't want to do.