Your feelings of frustration are not unusual and quite normal. As aikidoka we all have good and bad days in the pursuit of improving our aikido. That's what keeps me going back and continuing with aikido. Its the knowledge that I am continually learning about aikido and that there's so much more I can learn.
As for the feeling its all for nothing, yeah sometimes but then there's the feeling of getting something right, if just for a short while, that is sooo very satisfying. I live for those moments
Yeah progress sometimes seems like a standstill. I was in a dreadful rut one time but I managed to get out of it. I'm now training at two dojos with different senseis. Having a different perspective on techniques has made a huge difference. From this perspetive, Greg Jennings' advice is very sound.
More training isnt necessary the answer. For me I found that meditation, staying relaxed and enjoying aikido helped a great deal. There was less pressure to perform and get the technique right. For me, I do aikido because I learn something new each time I go to the dojo and not about getting to an end point. I think what Ghost Fox and Arianah say about relaxing is very true. I've found that when I'm relaxed and not over analysing the technique, the aikido becomes smoother and easier.
Hope this helps and keep up the training