What self-improvements have the most impact?

I am really into improving myself lately. There are lots of different things you can improve. I have been working on diet and fitness, procrastination among other things. But I also want to work on internal things. Things that will last my whole life. Also things that will cause me to improve in other ways(like self-discipline, helps you create any other habit or characteristic)

Can you give me some ideas and wisdom guys? Thanks!