It doesn't seem to work like you'd intuitively think - Working doesn't necessarily spend it - seeing results, winning, can fill it up. Drudgery, working on a problem with high stakes and no clear idea of the right direction, having your hard work thrown away - those drain it.
Meeting people can either drain it or fill it, depending on whether you're an introvert or not, how much you're listened to and how much you have to get off your chest.
Physical illness can drain it immensely. Eating, physically relaxing can fill it.
I don't know how to deal with it. How do I explain to coworkers and bosses why my productivity is low, set expectations around a physical illness that has lasted two years so far? An going fight with a neighbor or spouse? It's too hard sometimes.
I got nothing. Guess the only thing we can do is keep our chins up and hope for the better times to come soon.