> The older I get the less wisdom I find in any philosophy that means to teach how we should live and particularly how to avoid pain and unhappiness.

There are two limitations with this proposition. First; most of the useful "philosophies", by which you mean schools of philosophies, and by that we actually mean "wisdom traditions", have already been incorporated into many aspects of the mainstream culture that they are as invisible to us as the pair of glasses we wear. Participated in any Abrahamic religion? You've also made use of Stoicism. Done any CBT therapy? You have practiced being a stoic. Read Hamlet's "there is nothing either good or bad, but thinking makes it so", you've heard Epictetus. Quoted the serenity prayer? Quoted one of the central Stoic maxims.

The second issue is parts of any doctrine that gives a didactic, algorithmic way of solving life's problems, are not the bits related to wisdom. Think it like the static data of any tradition, vs the executable bits, in which an on-the-fly heuristic needs to be run to actually approximate the optimal. Wisdom traditions only aim to sharpen the heuristic strategies, not to hand feed ready made solutions, because the problem space is inexhaustibly complex and the environment constantly changes.

The "should" statements you encounter in Marcus Aurelius's Meditations, or Epictetus's Enchiridion etc, are not the ready made answers you should take to heart. They are the example outputs that give you the perspective to train your own heuristic. It increases your chances to recognize the life inputs that Stoic optimizations can be applied to.

The core formulation of Stoicism is actually very simple, avoid agency ascription errors; don't take on anything you don't have control over, don't relinquish anything agency you do actually have. Constantly focus on what you can actually do, not the shoulds, oughts, would haves.

> Pain is useful.

Disease states exist, in which one is stuck in pain that is not useful. Chronic pain, phantom pain, chronic depression etc. They are pain states that have lost their utility.

> I saw perpetually and unconditionally happy people. In a mental asylum.

Likewise, this is a disease state too. Unconditionally happy people cannot survive "in the wild"; it is not adaptive or relevant to be happy in face of all life inputs. That is why mania is equally serious condition as depression, even though one feels more egosyntonic.

> The short of it is: nothing is good or bad, but things can be too much or too little. Where the line is depends or circumstances and is different every time. And the correct answer is mostly only available in hindsight if it is at all

Good or bad already implies too much or too little. E.g the original meaning of "sin" is missing the mark. It is the residual error we have anytime we apply our heuristics.

If the local optimal point was only available in hindsight though, we couldn't have survived as a species thus far (not saying we are missing some better optimum and we will definitely survive). But turns out our strategies have been good enough (and getting better) to make us pretty OK general problem solvers. If our heuristics was so low precision, we couldn't even talk and understand one another, which requires mentalizing the state of mind of the recipient.