I'm not sure this will be very helpful to a lot of people (probably not), but it's the kind of thing that helps my silly brain, so I'll just share it. :p Hopefully it doesn't hurt yours. ^^
In statistical terms (don't quote me on this, it's very unscientific writing, lol), a "great" day is significantly different from your "average", it can even be like an outlier, an "extraordinary" day. This means it occurs way less often than the "average" day (depressive day?), so the likelihood of you having another great one on the next day is actually low. It's much more probable that the next day will be average again, and that may feel worse because you've just had a great one, or it can also be an actual "bad" day.
Life is ups and downs indeed. If you suffer from depression for example, the out-of-the-ordinary days will be the good ones, and they will be rarer...
Now what can you do with this boring and depressing information? Well... The good days will happen even if they're less frequent. It's worth waiting for them, remembering that they will occur, and most importantly enjoying them to the fullest while you're having them. In particular, if it makes you more productive, take advantage of it. These good days are what you're fighting for, and they can become more and more frequent, until the average day isn't depressive anymore, but more decent.