I cannot of course say what is happening with you. But here is a thought, since you have used the tag of impermanence for this question (and quite correctly, I think).
Our minds are addicted to sensations- craving of pleasurable sensations and aversion to painful sensations. The addiction is to react, depending on which type of sensation one is experienceing, which changes from time to time (hence the impermanence in the subtlest form). An addicted mind, like an addict, grows accustomed to a high- once staisfied with one drug (here 'fun'), it wants another. Addict wants a high and once that is satisfied, it will want another high. This wanting of high (craving) is a conditioned pattern of mind, at the deepest level- conditioned because of our ignorance to the impermanent nature of phenomenon.
What you thought of as fun, is now boring. But the mind is addicted to 'fun'. So now it is in a bad mood since it can't find something new, a new fun thing to do. Don't get me wrong, having fun is important. The obsession is harmful- we loose the balance of mind.
First of all, you haven't forgotten how to live. Do not be discouraged. The fact that you have that seen how hollow everything is sounds to me like a moment of insight.
It might be that you feel you have forgotten what really matters. But maybe you are also beginning to understand that you never actually understood what really matters.
What we know intellectually, is one thing - but we also must ask ourselves, whenever we learn something - how does it change my world?
If I really know that everything is transient like the form taken in a wisp of incense smoke, then it's hard for me to develop an attachment to their permanence, right?
In some schools of Buddhism, there are three types of laziness - they are opposite to the three types of effort.
The first is apathy lethargy: https://www.youtube.com/watch?v=fLexgOxsZu0
The second is a lack of transvaluation: finding no pleasure in doing what is good, or finding pleasure in doing what is bad. https://www.youtube.com/watch?v=hT_nvWreIhg
The last a lack of confidence or belief in your ability to do good. https://www.youtube.com/watch?v=HDqmJEWOJRI
It is this last one which is the most harmful to our path-treading, as if we believe we cannot do it, or that it's impossible, we end up living in a shit world with no escape.
The direct opponent to laziness is effort, with the corresponding antidotes to the 3 above.
But the underlying cause of laziness is wrong view. If we are able to see the world as it is, then the natural path of least resistance is to follow the path, so developing your insight into the nature of reality is fundamental to changing the way in which you behave within it.