The most important thing to understand about yourself
Learn to separate the real you from your feelings and your thoughts and your life will change for the better.
This headline is no small headline, but I'm confident that this is the most important thing to understand and it's a thing that has totally changed my life. I first read about this concept in the book "The Power of Now" by Eckhart Tolle.
For such a simple thing it's unbelieveable how powerful it can be. It can be summarized in the following picture:
You are not your thoughts. You are not your feelings. Also your thoughts are not your feelings, even though all of these can affect one another.
Learn how to separate these things and your life will change so much for the better.
Your thoughts come from a mysterious place and you can't control what thoughts come to your mind. You can however decide not to be controlled by it. If the thought is not useful in your current situation, you can just thank the thought and let it slide away.
Your feelings are also temporary and while it sometimes might be important to follow your "gut feeling", feelings can also be misleading. Like when you don't feel like doing something. Or you are scared of something that is new to you.
You can have values, the things that you want to pursue in life. You should decide those values. Not your thoughts or your feelings.