you lack focus or a calm state? if you lack focus, i could see why you'd grow tired trying to find a spell. if you feel drained when doing any type of research, or task, it's probable a vitamin deficiency, go and talk with your doctor. another reason for feeling lethargic could be a lack of exercise. doesn't explain why you'd grow tired after looking up spells, but they're options.
if you want to cast a spell for beginners, use a candle spell. take a candle of the corresponding colour [green = luck/health/wealth pink = love, white = peace yellow = friendship] then sit holding the candle, charge it with energy. light the candle and focus on your desired outcome.