I've found that Aikido has occasionally started to wear on me if I take too many classes per week over a long period of time. This problem is exacerbated if all my classes have the same instructor, or instructors who are very similar in technique and teaching style.
I would recommend cutting back to two-to-three times per week. I would also recommend (especially if you only have one instructor or are the instructor) that you re-examine your training style. Try to start learning from your ukes instead of your instructor, and try to read between the lines into what the technique could be instead of what is taught. That changes the tone up a bit, and has helped to get me out of ruts in the past.
I've also found that going to a seminar with an instructor who is sufficiently different from my regular instructor can re-energize me. Probably many of those cross-training Aikidoka can cite their other martial art as a similar boost.
The cause of this frustration and boredom, for me at least, seems to be that if I do too much training without enough time off the mat than I have problems making the changes in my technique and demeanor that the training leads me to in order to progress. I get too close to the problem, and I can't get around it.
There are often simply too many problems with my technique for me to look at them all in sufficient detail, and none of them are in bite-sized chunks. Not only that, but training overkill makes me see the whole mass as being undesirable, unattainable, or both.
For me it is better if I take my time and integrate changes piece by piece, and I can see things more clearly when I'm not beating myself into trying to find my 5th class that week by the same instructor new or interesting.
If I train too much, I also take Aikido for granted instead of finding it fun. This is probably the biggest reason why I never train while I'm on vacation - I like to come back from vacation and be really excited about coming back to the dojo.
(not at 12 years yet, but at 8.5 and still going)