Reframe all your negative thoughts.
I used to obsess about past mistakes. Thought: "I wish I didn't do that." Reframe: "If I didn't do that, I wouldn't have learned that lesson to be the person I am today. Or in the position I am today."
Stop worrying about problems that haven't even materialized yet and very likely never will.
We waste tons of mental energy worrying about problems that never happen. If you don't believe me, make a list of all you worries about near future events. Wait a week and see how many of them actually happen. You'll find 99% didn't.
This one was the hardest to put into practice: be kind to yourself.
A lot of people believe that if you stop criticizing yourself, you won't learn. You won't improve. You'll become arrogant.
When you criticize yourself, you actually make it even harder to learn from your experiences / actions. Why? Because when we feel badly, we're not very productive about forming ideas on how to change and putting them into action. Instead, we seek distractions (sometimes really unhealthy ones) to try to make ourselves feel better.
When you do something that leads to a negative outcome for you, preventing that negative outcome from happening again is enough motivation to change in of itself. You don't need to make yourself feel bad to learn and change.