I, too, have experienced a severe decrease of my old interests, which were wildlife and drawing. These interests sort of faded away when I reached puberty. In my late teens, after graduating from secondary school, I have repeatedly attempted to revisit and reawaken the old spark for my old interests, but with very little success. I attempted to read about animals and plants, I tried to draw a little bit; and yes, I did enjoy doing this to a certain extent, but... it just wasn't the same, wasn't as intensely felt as it was when I was a child. In the meantime, even as I continued to muddle on with my old hobbies/interests, I also introduced myself to other, new things. I started getting a little bit interested in East-Asian popular culture, starting first with Japan, South-Korea, and China, and then averting my gaze southward to Indonesia, which is also the country where my grandmother was born. My interest in Indonesia was also strengthened through the good rapport I shared with my Filipina colleague at work, since she would often tell me stories about the Philippines, which share cultural similarities with Indonesia. And now my interest for Indonesian culture and history is pretty much as full-blown as my old interests for dinosaurs, drawing or Transformers once were. I'm by no means an expert on the subject, however, I'm still pretty new to all of it, but that's also what I love about it.
The point I am trying to make here, is that if you are worried that a continually decreasing lack of interest is leading to apathy and depression, perhaps it's not a bad thing that you're losing your old interests. Perhaps that's not where your salvation lies anymore.
I, too, have felt despair when I realised that I was starting to feel depressed, and could no longer turn to my old interests for comfort, because they weren't doing it for me anymore. I managed to chase the blues away temporarily by listening to light music, by watching anime DVDs, by visiting my relatives and in particular my mother. But at the end of the day, I realised I got one life to live as the cliché goes. Since I was unemployed at this time, the first step I had to make for myself to feel a bit more secure, was get a means of livelihood. Think about the primary needs first: eating, sleeping, breathing, paying the bills. After a while, though I find my own personal time, and the intellectual and creative pursuits that belong there, far more important than a career and a job (many people are of the opposite opinion on this), I was able to walk around the streets with the notion: alright, right now there's nothing else I have to do than just be, just exist. Breathe in and breathe out. While your interests are very important, the specifications of those interests are of secondary import- the important thing is that, before you pursue anything, you feel comfortable about yourself.
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clarity of thought before rashness of action