5. They know how to say no
Mentally strong people know when to draw the line and say no. They accept that it is healthy and normal to put yourself first and have the self-esteem to refuse things and be clear about it. On the other hand, people who cannot say no are usually more stressed out and depressed than those who do. Having control over your actions and decisions is an empowering feeling and saying no from time to time can be very liberating.