Another very valuable post. And to be honest the fact that you know what you have to do and you have been challenged (and absolutely no doubt your diving will have improved) so you have gained from it.
If you want someone to go play with who has seen a whole lot of attempts at shutdowns then give me a shout. I'm sure the GUE course and Mark's course will have identified what you need to be doing to get there, keep working at it and keep diving.
Don't forget to go diving where you don't even think about shutdowns. If your diving is all about getting it shut down and you're not getting it in a hurry it will be demotivating, which isn't what you need. Just shut the isolator and dive indies and do fun diving where you can. I have had gear problems that pissed me off for ages and I found a way through it, but you have to make sure you don't forget why you're working so hard in the meantime.
This is especially true with rebreathers that keep trying to do you in no matter how much gaffer tape you use
Digs.