Sheleth has two big insecurities:

1. She doesn't think her war-torn, environmentally decimated planet deserves to be in the Federation. (Not true - the Federation's standards aren't as high as she thinks. Tellarites are a member but in Journey to Babel they wanted to keep another planet out of the Federation so they could mine it)
2. She doesn't think she deserves the rank of commander, and they only gave it to her so they could have a Kasheeta in charge of a ship in Starfleet (This actually is true, but it's not that unusual for position to determine rank)

Also, Sheleth's body shape apparently depends on camera angle.