Why do I bonk so easily?

Could you possibly enlighten me as to why I seem to bonk quite easily? Is it perhaps due to some underlying physical factor that I'm not aware of? Or could it be psychological, perhaps a lack of confidence or a tendency to give up too soon? Could diet or exercise play a role in this? And if so, what specific adjustments should I make to improve my endurance? I'm keen to understand the reasons behind this and what steps I can take to overcome it.