My 8th grade teacher had this poster hanging up in his classroom.
Mantras are a powerful thing! What you think about is the first step to forming your destiny! It also helps you to have healthy thoughts about yourself and to feel empowered on a daily basis.
You should chose a mantra that has the most meaning for you.
My mantras are: I am strong. I matter. I am loved.
I always say them put together because it adds more power and meaning when I say them all at once.
Here is how I chose mine. One thing that is important to me as an individual is feeling that my contributions are appreciated and make a difference. I like to feel that people are paying attention to my feelings, thoughts, and efforts. I decided that I could shorten this to say: I matter. Another thing that is important to me is feeling loved. I love to feel that no matter my imperfections, people support me and care about me. Loyalty and true love are important to me. I also love to be adored in romantic relationships. I don't care about having a million friends, but I value quality relationships that matter. I can sum this up into a mantra by saying: I am loved. Another mantra that I chose was "I am strong." I chose this one for different reasons than the others. I chose it because sometimes I don't feel that way, and saying it somehow reassures me that I am strong.
Figure out your emotional holes and what you need to hear to be reassured, comforted, and feel happy.
Other possibilities or ideas include:
I am capable. I am beautiful. I am enough.
I am talented. I am brave. I am kind.
Choose something that has the most meaning for you.
Literally, just reading the words in my mantra daily makes me feel empowered and brings me joy.

Another fun idea that you can do with your mantras is to engrave them into jewelry. I saw this idea from a friend. At Christmas time we were exchanging presents and one of the ladies had gotten a bracelet from her daughter that said, "I am strong. I am beautiful. I am enough." I longed for a bracelet with mantras on it. I thought that if I could just get one like that that I really would be brave enough and feel confident enough when I wore it to go on with my life at that point in time. It offered so much hope for me. So I got one made! And I gave one to a friend.
The website is https://my-whys.com/. I chose the 14K gold bangle bracelet to have engraved.