If you were to tire that quickly, it would largely be breathing technique. I had the same issue for my first and second fight, but corrected it for my third. The main problem with being nervous is how the body will tense up, which includes limiting the breathing.
From my experience, Thai kicks are one of the most energy consuming attacks in martial arts, and perhaps it is why such a distinct breathing technique has grown in Muay Thai.
Strong abdominal muscles are very important to have when it comes to your armor against knees and body punches. An over abundance of muscle can certainly limit one's breathing and movement in extreme cases, though if you are building that muscle in a non-stationary way, you should be able to maintain looseness. ( When I do pushups I usually accelerate on the push upward to mimic a punch).
For the breathing technique, you want it to develop naturally. I believe to learn it, you have to push yourself to an exhausted state and begin to vocalize your exhaustion as you push on. With every breath out you can say "ha". This will be easier and less awkward when you are already gasping for air (lol). The point of this is just to emphasize communication toward yourself, to your body, and to your training partners. It's easy to get lost in your head and lose focus on breathing while training, so the goal is to overemphasize it vocally and keep it conscious. This should give you more endurance, and keep your body loose and free flowing. Always remember to breath out on your strikes as well, whether it's a slight "shh" or a "HIYAAAA"