You said:
"I can only afford one lesson a week."
I'm there with ya!!! $$$ is tight.
BUT, as one of my old instructors used to say, you only have one or two classes a week to learn, but you have 7 full days to train. It's with your instructor that you learn, it's on your own that you get 'good'.
Things to keep in mind for 'energy levels':
#1: your age. We don't have as much energy (non-cultivated) at 35 as we did when we were 18. Must face this, but don't ever let it get you down. I've known FINE martial artists who started Karate in their 40s or 50s, after YEARS of smoking and abusing their bodies.... then they just took off physically in the martial arts! Take age into account, but don't let it count you out!
#2: Rest. Our time in deep sleep is when our bodies and minds generate our stores of energy, and only then!!! IF you don't get enough sleep, you won't get enough energy and your working out will only deplete..not cultivate. I learned this hard and heavy in body building... if you don't sleep you can't recoup from your workouts! PERIOD. It's a matter of quantity.
#3: Diet: our bodies draw ATP (the chemical needed for muscular contraction) and calories/carbohydrates from the food we consume. No fuel in, no energy out. It's a matter of quality.
#4: get at total of at least 3 hours of cardio work each week. Interlace this with some sprinting to help your body generate sudden bursts of energy.
Remeber to make your workouts mirror the type of exertion that you will later demand of your body. In other words, if you wear out sparring... spar more, or spend time shaddow boxing (and under rated habit, well worth developing). The best way to improve your endurance is to demand more endurance w/in your particular type of work.
Do your art more, more often.
Just some thoughts and suggestions.
Your Brother
John