This is the answer.

Our brains are designed to ignore ordinary things. When you drive the same way to work, do the same things at work all day, come home and ready a familiar meal, then veg out in front of the TV, you've done nothing memorable all day.

Then, in retrospect, the days seem to fly by because you don't have any memory of the time passing.

If you deliberately inject novelty into your days, then you'll have more memory of the events, and your days will seem full.

The other ones here related to memory (journaling both for being mindful of your days but also to reread and trigger the memory retention curve) will also help. I haven't seen sleep mentioned, but good sleep is also key to being alert enough to encode memories, having the energy to try novel things, and rest enough to store and process memories for longer-term retention.

Research also says a 20 minute mid-day nap can help with memory formation (not 50 minutes; a full sleep cycle will make you groggy in the afternoon.)