I have no advice but feel the same. I'm not really old, but feel there is nothing to live for, look forward to, and life will just be downhill, and also your body gets worse and worse as you age, pain illness, so then there's THAT to look forward to (looks I don't care, was always ugly). I also have conditions that make me not able to be part of the dating market. I've accepted being alone forever, however.
And the focus on the little things. That only kind of helps me. Cuz wtf--so your whole life was savoring a cup of tea or a sunset? Could be worse I guess.
I do like you op where I just distract myself from life and myself: with books, movies, tv, videogames, cleaning, hobbies like crafts sometimes. It still feels rather empty though sometimes. Like yay, I watched tv and read all life, whoopie.
I know lots of people say go to a church or other religion. That's not for me either. I do sometimes think about what I believe about various things. I research lots of different ideas cuz it's interesting even I don't come to conclusions. Maybe pick some topics, not just existential, to research?
I think I came to the conclusion: ok, so if the rest of my life is alone and doing nothing but tv, read, games is that that bad? I decided it's sad but I've been through WAY worse shit in life and would rather be where I'm at now than deal with some other crazy shit I have before, or could be.
Maybe something you read or see will spark something for you...maybe the timing just hasn't come yet?
And yes you could have a relationship if you want one/it's not impossible. I have disabilities that shove me out of the dating pool. It is a legit concern. People who say it's not are in denial. It's not impossible though. I decided I do not want any relationship whatsoever so I do not pursue it and have made peace with it. Being alone has its perks too.
Try and search your heart, not your head. (i'm not good at this, but sometimes I am, and it seems to make a difference to my well-being either physically, emotionally, spiritually).
I am not a people person so loathe volunteering, but that is a good idea as far as a purpose for some. Do you like animals? Do you have a pet? Won't solve everything but might help. I got a pet during lockdown, had always wanted one, never did, but finally did and that has helped me a lot cuz my pet is my only living friend.
and a lot of young people get chided for feeling this way. Like you are not allowed to. And the answer is usually, oh you are young, don't feel that way. End of. Period. BS. Older people don't have the monopoly on these feelings. And some younger people spend the rest of their lives like this, so it is a legit thing. I've known older people who say they spent decades like this. So I wouldn't ignore it as childish youth angst. Try and search your heart, and not society's chatter or sometimes your own mental chatter. I have yet to figure it out so rofl at me for saying that, but I believe it even if can't do it lol.
it does feel like I just every day try and kill time. Just trying to distract til I die, and that's exactly what I'm doing, which is sad, and I have little ideas, just empathizing. It's such a soul-crushing, all-consuming ideology--that it only gets worse and you suck and it's just life now. Some things are acceptance, can't change, some can't. It's hard to know. And yes, with all the covid shit, life as we know it has changed on many levels for everyone, even internally, and it's denial to pretend you can supersede this very real world-wide issue completely.
but hey, there are a lot of good books and movies etc out there so let's not knock em 
Hope you find peace, serenity, joy one day
. And hope you feel you deserve it, cuz that could be partly it too? (I know I on a deeper level feel I don't deserve shit so there's always that...)