Books about learning to be at peace with yourself

Back in my school I had this social studies' teacher who was also my mother's colleague. My mother was very impressed by her and frankly, so was I. She was the type of woman who could spend hours in only her own company without ever needing anyone else's attention. And I've always seen the difference in someone's energy when they're by themselves because they have no choice vs when they're by themselves out of their own choice. She was so self assured and had this air of confidence that didn't seem overt or anything.

And I'm the opposite of that; I overthink, I worry, I find myself to start feeling insecure if I'm ever by myself. And I've recently joined med school and my classmates are.. well, it is to say I won't be having any close friendships. And I want to better myself, to feel at peace with my own existence and not require other people's company to feel fulfilled. I just don't want to be this insecure mess. I try every day to better myself and I am seeing progress, but I'd like to know if there's any literature that could also help me.

Any suggestion would be appreciated greatly 🙏🏻