15 years doing anything will bring monotony, IMO. Doesn't seem to be a pathology problem, more of an existential one, but agree that as we age and increasingly feel our mortality, the daily onslaught of cancer can be depressing, or at least 'not particularly uplifting'.
You've spent your life (presumably) on the usual 4-6 yr cyclical trajectory: study, work hard, achieve goal, move to the next level. Now you're playing 'creative mode' and it gets boring. Entirely relatable. You've been practicing for 15 yrs so we're not too far off. How about these for a start:
Malcolm Gladwell's
Outliers. Maybe you've already read it. If not, maybe it will provide the notion that doing something for 15 yrs (or longer when you include all the prerequisite training) makes you an expert of your craft...maybe not a world-renowned athlete or Nobel prize winning scientist, but you've honed a craft that puts you in a unique position relative to other people.
Jonathan Haidt
The Happiness Hypothesis.
The Happiness Hypothesis - Jonathan Haidt
He has some other great books as well, particularly as to how you should be grateful you're a part of late Gen X / early Millennial generation and avoided a childhood filled with limitless technology / smart phones and the dangers therein (eg.
The Anxious Generation and
The Coddling of the American Mind.)
I mostly read non-fiction history-type books which may not be much help aside from providing historical perspective, which is generally good. And while I understand that the frequent "look on the bright side" response can be toxic positivity, there's never really a better time to take a step back and appreciate one's position in life than when you're in a rut.
Or maybe try doing something that challenges you for no reason other than it challenges you (goals and endpoints are often helpful). Try learning a foreign language. It may sound lame, and it's certainly harder on your own vs in a classroom, particularly if you're lacking motivation, but sometimes when you get started it can provide motivation in itself, especially if there's an endpoint or 'deadline' (like a trip or vacation). I took Spanish in high school and college and didn't find it particularly enthralling at the time, but that was 14-20 yr old me. Currently using Babbel and reading a few books to learn Italian, which serves no purpose other than to make the prospect of travel more enjoyable...and to impress my wife TBH. 30-60 min a day isn't too bad, or at a minimum, one lesson on Babbel. Will take longer but repetition is repetition.