What does she mean by saying "you've been working on yourself"?

Native speaker’s answer
Rebecca
To "work on one's self" means to improve yourself by bettering the way you think and how you do and manage things. This could be mentally, physically, spiritually, relationally, or otherwise. You work on being better in those areas. So here, she is saying that he has been working on being a better version of himself! Ex: This year, I want to work on myself. I'll become a better version of me. Ex: Rachel said she's going to therapy to work on herself.