Meditations, by Marcus Aurelius. Bottom line: judge your success in life by how well you make your decisions, not by your outcome. You have full control of your decisions, and often no control at all over their results.
Is an outcome not the other side of the decision-making coin? I understand that the outcome should not carry all the weight, but it surely often carries a lot of the weight of a particular decision. That is, how does one get good at making decisions? By seeing positive results flow from them.
No, and that's the key. You cannot truly control outcomes, so judging your life and self-worth on them is leaving your happiness and fulfillment to chance.
> That is, how does one get good at making decisions? By seeing positive results flow from them.
Nope. You get better at decision-making by being reflective about your past decisions. Outcomes can be a factor in that, but only insofar as they can point you to the realization that you missed information that was available to you.
There are many decisions in life where there is no "good" outcome. There are some where the "good" outcome is catastrophic for you personally, and that catastrophe is avoidable if you compromise your ethics. If you see a child drowning in a river and know that there's a good chance that you will die if you attempt to rescue them, rational self-interest alone tells you to walk on by. Stoicism puts a layer on top of that - can you live with yourself without regret if you do that? Are you willing to accept the risks to live up to your own standards?