I've only been in the martial arts for a bit over a decade.
Although I have trained in Kenpo/Kajukenbo (well, the program is kajukenbo, but it split before the name Kajukenbo was used) over that time, I've supplemented that training with focused classes on the side. I think if you stick with one group exclusively, you converge on the club's fighting and training styles. While such training is very focused, it leaves you very well prepared for your dojo and less so for other areas. So, to become
better at sparring, I studied some Muay Thai kickboxing. To tighten up my sense of space, I studied some Pentjak Silat. To learn weapons, I studied Filipino Inayan Eskrima. To learn how it feels to really have a stick swung at me, I fought at the Dog Brothers stick fighting Gathering. It's useful to supplement your class material with books, videos and seminars from other styles.
As for physical training, any sports training book will tell you about cyclical training. That is, you build up a training base, build-up to a peak and "build down".
You can't be in peak condition all of the time. Strength
and endurance are opposites. The former trains all of
your fast twitch muscle fibers to fire at the same time.
So, you are phenomenally strong for a few bursts.
Endurance training trains your slow twitch muscle fibers
for capillary growth and your fast twitch fibers to fire cyclically. So, you are not as strong, but can last.
Until I understood this, I would train to fatigue or failure.
More poorly executed reps during fatigue no longer dominate fewer, higher quality reps with more rest.
Training is about stress and rest. Many forget about the rest part, get injured or burnt out, then take a lot of time off for an injury.
The burn-out that you mention is a well-documented phenomenon.