Perhaps there are different kinds of 'enjoyment'. And perhaps I'm personally willing to accept some immediate suffering because I know I'm pursuing a greater enjoyment.
I'm certainly not advocating for (constantly) pursuing in-the-moment unbridled hedonism.
Something which may be related to this is the idea of type 1 fun and type 2 fun. I'm not sure where I heard this, but few people I talk to have heard of it.
Type 1 fun is the kind of fun which is enjoyable in the moment. For example, computer games and watching youtube videos.
Type 2 fun is the kind of fun which is enjoyable upon reflection but not necessarily in the moment. For example, running a marathon, certain moments of trial/despair when working on a big project.
When I first played Factorio I played it for about a week solid. I loved it. After that I could put it down and do other things. I now play it for a couple of days every couple of months when the mood takes me. This is type 1 fun for me.
I'm certainly glad a ran a marathon, and it is a very happy memory, but no way in hell am I going to do that again. Very much type 2 fun.
Some comments (not necessarily the one I'm replying to) seem to inversely correlate enjoyment with how hard a task is. The harder a task is the less likely it is to be enjoyable. Is this true? It certainly isn't my experience, but maybe I'm odd as I originally suggested.
Something which has also driven me is boredom. Being a freelance developer has given me a lot of free time. Certainly for me, type 1 fun only goes so far in conquering that boredom. Sure, play computer games for 6 months, but then that gets boring too. At least it did for me. Then what?
Then, for me, type 2 fun starts to look more appealing. Perhaps I should build a house? Sure its going to be hard, but I'm practical and I like leaning new stuff. Plus I'll be outdoors, and at the end of it I'll have a house. (FWIW, I actually did this)
Perhaps that's just being an "up" kind of person. I think it is also believing in one's abilities.
I don't think this is a cohesive point with a strong conclusion, but perhaps this series of paragraphs is useful in furthering a discussion.
I'm also "ADHD." Quotes because after 30 some years living with the diagnosis (and reconfirmed, both by others and myself), I still don't really know what it means. Something sometimes discussed is "hyper focus" for ADHD folks. Honestly I think this is just a normal expression of that other quality: a total inability to engage with something once it becomes boring. And excitement for those things that retain interest. Naturally, things that are confusing or complex tend to be impossible to initially approach, but gain momentum as you gather more clues and understanding.
I have great sympathy for those that try to go "the hard way." I did that before I got into software development. I denied my misery (of not software dev) for a decade because I understood that hard things are hard, and believed my misery would pay off.
Turns out, I didn't have all sides to the equation. Hard things are hard AND hard things that you are passionate about can be your way to contribute to the world. Doing something that is "hard" BECAUSE it is hard only because you have no passion in it is a bizarre but plausible scenario for what I imagine are many HN readers.
If instead I go deep on what I enjoy, I am happier in the short term AND long term. When I was younger, I would "waste" time on strategy/tactical games. But I think I actually gained some long-term planning skills from this, and developed a taste for "slow thinking" from this activity. And now those games are boring to me.
Last weekend I buried my head in functional/algebraic programming, even though my conscious brain was saying it was a total waste of time and distraction from my real tasks/goals. From the experience, I've learned that functional programming really isn't the silver bullet I want to believe it should be. But I trust myself enough to believe that even if the payoff isn't obvious, or even if it doesn't apply in my professional life, that my brain giving me enjoyment for the activity is enough of a signal that it was time well spent...even if I really can't justify it to anyone including myself.
You can point yourself in a general direction by setting up your environment towards that direction. Buy books in things you're interested in even if you never end up reading them. Give your body sunshine. Never go grocery shopping when hungry, and fill your fridge only with good foods. The key to letting your unconscious brain take over is that you only give it access to the sugar that your executive planning brain believes will work.
You should to some extent enjoy what you are doing particularly when what you enjoy is in line with what you want to get out of life.
The OP asked for tools for focus. Most of us need tools to gain perspective because we are so busy day to day that we don't see how it will fit in with a full life. We all need tools to periodically take a step back and see how everything fits together. I think you do that naturally but it was not in your post.
BTW - I can tell you do not have young children. They will completely change your perspective.