I went through this exact same thing in my late teens and my adult-but-not-really-adult years, and continue to have the same thoughts even today. I just want you to know that these thoughts are natural. Creative types go through these cycles all the time, though it’s important not to endure this alone.
For me, a lot of it has to do with the development of the inner critic. As we get older, we get better at what we do, but our world gets bigger too, so we don’t really see our own growth in comparison to what others do and how much better they do it and how can we ever keep going if someone can do it better and—etc. It could be that your lack of motivation is a side effect of something bigger like this. It was for me.
I’ve also heard that, for creative types, they hit a wall when they realize they’re not as good as they want to be. The internet, bless the thing, makes this worse, because now we have greater access to a massive volume of people who write just as well or better, with more original ideas and so forth, and it’s pretty damn disconcerting. That might be part of your block too. It’s a lot of weight to carry.
Or, it could just be that the original reason for your writing changed. When people say they need to be depressed in order to write well, I understand, because writing used to be the best therapy in the world for me. But, after awhile, I didn’t need it for therapy, and I had to rediscover why I loved writing in the first place: I just liked to tell awesome stories that got people as excited as I did.
Anyway, enough about the possible whys, since these could be totally wrong. Here are some suggestions for you:
Good luck! I’ll be cheering you on!