Sheleth has two big insecurities:
- 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)
- 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.