I am motivated by my emotions processed by my reason. I think about how to improve things for me, for people around me, and for people in the distance. That is sufficient to keep me moving. Not sure if this answers your question.

In the meanwhile, although I expect AI to be a game changer, I guess I am mostly working for the Everett branches where life somehow turns out to remain normal?

If we somehow get the good kind of singularity, my meaning of life will be to have fun. There are lots of things I would like to do: read books, watch movies, play computer games, learn all kinds of things, etc.

In my life, there is almost zero connection between "the meaning of life" and "a job". I see the jobs as a necessity, but I identify with the things I do in my free time. At work, someone else is the boss (I don't have the skills necessary to start a successful company), so ultimately it is their right to screw it up in any way they choose to. I try not to get emotionally invested in things that are beyond my control.

To be honest whether one feels fulfilled is much more about emotions than any logical argument about one's global utility in my book. If the opportunity arises leaning into personal relationships (both romantic and not) can give a strong sense of belonging. (Try it if it comes into your life! Really!)

I also have a reading list of myself and other people's recommendations that might help you. These are books about possible social change/solutions to world-scale issues/hope in the darkness. DM me if you're interested.

Having kids is a huge part of it for me. Came by it but absorbing a philosophy where it mattered a lot from my parents, which I liked and accepted; I've always aspired to be pretty similar to them. Now I've got a baby and yep, as I hoped or better. 

I value many other things too, which have been pretty stable since high school. Helping the global poor somewhat, creating art, having friends, eventually getting married and being a good spouse, and some idiosyncratic imagination stuff. These things have all seemed obviously good to me for so long I don't really know where they came from. Just following gradients as a kid and imprinting on (I think good) role models, maybe. I think I endorse it all, though.

I doubt this helps your particular quest, but that's my answer, at least right now!

I came to the conclusion long ago that meaning is not an objective measurable thing - it's a personal experience, on the order of qualia (may actually BE a quale).  People attempt to describe their experiences and beliefs, but only imperfectly and immeasurably.

For me personally, I've found curiosity to be a big driver - I wonder what's going to happen.  I wonder how this seemingly-unimportant product can improve someone's life by a little bit.  I wonder how this group of conflicted imperfect humans can actually cooperate well enough to find and execute toward a shared goal.

I try various things, and some seem to result in more pleasant and successful outcomes.  There's a lot of variance, which keeps the uncertainty and curiosity at the top of my mind.

This mixes well with a sense of beauty, pleasure, and wonder at some things - I really enjoy both natural and man-made environments, and the actual sensations that come with being in various places.

 

I'm new to the lesswrong community, but I'll share my 2 cents. First, my sense of meaning has always come from an appreciation of beauty in the world. This includes a fascination with the natural world, mathematics, music, sunsets, food, etc. (Occasionally even people!)  Second, my life became much more interesting (read: meaningful) when I left silicon valley. I think there is something soul crushing about building over-engineered solutions to first-world problems of dubious relevance to anyone. I quit my job working for a Blockchain startup in 2018 and switched careers - now I'm still coding a lot, but I do it in the context of medicine and cancer biology, and my skillset is now much broader than code. I don't think this is the only way to increase a sense of meaning, but I think it's about finding ideas that genuinely excite you, and choosing what is interesting over what is easy (my life sucks in a lot of ways, but it is more meaningful now). I don't know how much this is helpful or answers your question, but this is what I have found so far.

what am I missing such that I don’t have it?

Probably a model of this stuff compatible with your general outlook. The sordid truth that various mystics and spiritualists don't want you to know is that there's nothing particularly cryptic or profound about "meaning". If you have anything worthwhile to get out of bed for - that's meaning. If you ever decide to do anything at all - that's meaning. If you'd prefer doing something, but find yourself doing another thing - that's meaning too. Of course, figuring out all those priorities and tradeoffs can be complex and confusing, but introducing the word "meaning" into the mix couldn't and shouldn't create any additional complexity.

I found Meanings of Life by Roy Baumeister very helpful to sort out what exactly to expect from meaning of life and how to get it. It is somewhat interdisciplinary but primarily a psychological study of what people who have a sense of meaning in life mean by it and how they acquire it. In style, it reminds me of a book-length Much More Than You Wanted To Know post on ACX.

"What is the meaning of life?" 

I've always viewed this as an unclear question. I have no idea what kind of answer one might be satisfied with or even view as being legitimate as a response. I think there's a couple of questions rolled into one with 'what is the meaning of life'. 
 

1. "What is the meaning of life?" = "What should I be do with my life?"
2. "What is the meaning of life?" = "Is there a non-contingent reason for me to act in the world?"

      3. "What is the meaning of life?" = "Does the universe/god/nature/etc have                              intentions about how I should behave"

For myself, I think my answers to all three have a certain similarity. I think this is true for most people and it's why they get combined into one question about the 'meaning of life'. Enough preamble. My answer:

I am a particular kind of thing. I am a thing that has interests that are inherent to being the kind of thing I am. I try to act in accordance with what is 'good for' the kind of thing that I am. I often fail, but hey-ho. C'est la vie. 

Less BS - I gain most of my meaning through conversation, goals which lead me to learn about stuff, moving towards a job that gives me a decent amount of freedom and that interests. Most importantly of all having fun. 

On a personal note to you, dear Aprillion : I think having a 'goal' is perhaps the most important thing in terms of 'having something'. The second you start moving towards something you genuinely desire, even if the goal is deeply unrealistic. That feeling of meaninglessness disappears. I won't say I have some deep sense of 'meaning' in some spiritual or zen sort of way. But, I definitely don't think things have been meaningless since I've started to move towards things I fundamentally wanted.  

I think Sam Harris had the right idea when he said (don't have the link unfortunately) that asking about the meaning of life is just bad philosophy. No one who is genuinely content asks about the meaning of life. No one is like "well I feel genuine fulfillment and don't crave anything, but I just gotta know, what's the meaning of everything?" Meaning is a thing you ask about if you feel dissatisfied. (And tbqh that's kinda apparent from your OP.)

So at the real risk of annoying you (by being paternalizing/not-actually-aswering-your-question), I think asking about meaning is the wrong approach altogether. The thing that, if you had it, would make it feel like you've solved your problem, is fulfillment. (Which I'm using in a technical way but it's essentially happiness + absence of craving.) I'd look into meditation, especially equanimity practice.

That said, I think re-framing your life as feeling like it has more of a purpose isn't generally pointless (even though it's not really well-defined or attacking the root of the problem). But seems difficult in your case since your object-level beliefs about where we're headed seem genuinely grim.