Depending on how you meant "I have been training for a long time", I may have been training far less long than you, so take my advice with a grain of salt and all that.
But, for me, I found the idea of treating aikido, and indeed any martial arts practice as self-defense, a philosophy, a way of life, etc. etc. was basically destructive to me. It was the very fact that I put such monumental importance that precipitated burn-out and a lack of enjoyment of it. I couldn't just go in and practice for the pure fun of it; 'cause, ya know, every interaction had to be deadly serious or I felt my practice was "wasted" or not "real" enough, or that if I wasn't doing what so-and-so was doing or training so-and-so amount that I was failing at elevating my practice to whatever pedestal I was putting it on.
At some point, well, I just stopped doing that. I stopped looking at aikido as self-defense or "fighting" or "not fighting". I stopped worrying about the philosophy of it. I just went in to practice for the simple sake of exercise and interacting with people. And at the same time, I was suddenly free to be more adventurous in what I was doing because the outcome did not matter. I could go off on tangents, and if they took me nowhere, so what? Eventually I just put my aikido training on hold, and I went off to new places that had that whole "shiny newness" again (in the words of the above), still martial arts - because I still like them on some level more than other sports, but I was no longer afraid to let myself suck again and learn new things.
So maybe if you can stop looking at aikido in terms of any purpose and just accept the practice at face value for what it is, a practice of some movements with other people, devoid of any meaning beyond this, it might help you work through it. Or maybe you might find you're happier just learning gardening and putting your practice on hold till it interests you again, why make training torture if you're just not that into it anymore?