Exercise: I used to do all kinds of programs incl "45 mins a day, 6 days a week" but found "sufficiently enjoyable results" while keeping the process enjoyable and in full balance with the rest of my life with just 3 reps a day, no break days, cycling through 8 exercises. Good enough for both health and looks if no Mr. Olympia goals.
For learning, the right balance is a lot harder to strike IME. Going too hard on oneself or too soft is a real danger. The motivating goals for learning something or other will have to be consistently and sustainedly present to settle into the right balance over time. Whether (perceived) low stamina is merely due to "the undertrained stamina muscle", mismatch of expectations and results, or some deeper real physiological/psychological factor is also the the likeliest found out by yourself. Not reason not to go meta on this roadblock!