Did both, I'm fine. The urge to label pessimists as mentally/physically ill instead of really tackling the arguments is understandable but quite annoying :\

There is the saying that if you can't explain it simply then you don't understand it well enough. In that complexity lie a bunch of implicit assumptions, and if you haven't identified them then you may come to see the conclusion as irrefutable while it really isn't so. — leo

You confuse between simplicity and length, Sam Harris explanation isn't complex - it's just long. That's the TL:DR:
A choice can be made in two ways and two ways only, algorithmically or random. I challenge you to imagine any other way of making choices. You can't. Neither algorithm or random is "free", it's either arbitrary or predetermined.
There are a bunch of experiments that show that your body starts to implement the choice before you even know about it consciously --> means you couldn't take it consciously.

What does it feel like to be eaten alive? — leo

Watch this (NSFL) Does it seem like the gazelle doesn't feel a thing?

Many of your other questions and arguments are answered in the article I've linked to regarding wild animal suffering.
Regarding human suicide:
1. As Viktor Frankel said, “Those who have a 'why' to live, can bear with almost any 'how'. Religions, false narratives, and ignorance helped people to create meaning in the world.
2. Suicide is very hard, the life instinct is very strong to override.
3. For some people, life is actually happy if they have the right genetic luck.

No free will - Quite long, but it's a complex issue.

When I say I believe in rationality I mean only towards what I think is true or false in the world, I don't claim that this is the only modus operandi one should have, but if you will ask me to accept truths that don't make sense it's something I cannot and will not do.

Regarding wild animal suffering.
"The total amount of suffering per year in the natural world is beyond all decent contemplation. During the minute that it takes me to compose this sentence, thousands of animals are being eaten alive, many others are running for their lives, whimpering with fear, others are slowly being devoured from within by rasping parasites, thousands of all kinds are dying of starvation, thirst, and disease. It must be so. If there ever is a time of plenty, this very fact will automatically lead to an increase in the population until the natural state of starvation and misery is restored" - Richard Dawkins

You can read here more about it, it seems that Wild Animals suffer immensely.

That's obviously not true. Hence why you're here. — Alan

Agreed, No narrative that I heard until now convinced me.

I don't really stick to them, the only thing I stick to religiously is rationality in the larger sense. I believe in things that make sense. Do you feel I'm wrong in any of my assumptions? Can you prove me otherwise?

Regarding why I focus on the negative: I don't, I look at the world as a whole - and the Sum seems to be negative. I do understand that for other people it seems positive, but that's obviously a completely subjective view. I see nature and how wild animals live and I'm disgusted, I'm not sure how to change it.

What made you stop believing this? How did you want to make your life better? How would the world need to be for you to be happy? — leo

Nothing, Happiness is just bodily sensations it's obvious the world outside won't make you happy. Maybe wireheading will help? Maybe other things? I don't know - I've tried many things in my life but I still felt generally somewhat unhappy. Why should I assume I will succeed after years of trying and failing?

I feel like if I had a sense of meaning or Ideal driving me forward I could go through the daily grind, but no narrative can convince me anymore.

You might enjoy this lecture by Peter Watts on the exact question you proposed.
The TL:DW is "No one knows, maybe consciousness is a parasite?, anyway it's a really good lecture and I highly recommend watching it entirety (don't be discouraged by the low amount of views)

I'm pretty well traveled, been to like 20 countries.
That's another issue: I feel like I did all the big things I wanted to do in life.

Can you say more about how you went from having ambitions and direction to feeling that everything is meaningless? — leo

My ambitions had to do with the belief that if I'll make my life better I would become happy, I don't believe in this anymore. In terms of the feeling that nothing has meaning - the blame goes to philosophy, science, and history. I feel like going thoroughly into all the theories will be too complex. The most compressed version is: Life is an evolutional process of entropy acceleration, there is no free choice, it's not even clear why there is consciousness, evolution is cruel, stupid and wasteful by design, nothing you do matters in a non-local scale, the "meaning of life" itself is nonsensical logically (Wittgenstein)

I don't have issues with death or the inevitability of death, I have issues with life itself.
I'm not taking any medicine, I'm also not depressed - I live a reasonably active life. Just neurotic.