While Oz, mentioned above, captures bravery rather well, it does not particularly score high on the spiritual side of the meaning of fortitude. Pretty name, though.
Eitan, as in "he who has fortitude" is another masculine name and can mean strong, brave and strong-hearted, as well as "there to stay". For example, Nachal Eitan is a river always flowing (does not depend on seasonal changes).
Amatzyah, mentioned above, possibly captures the spirit of the word fortitude best, but is also associated with God and is most commonly used in religious circles. Nothing wrong with that, but it may sound a little strange to some.
For a girl the closest I can get is Azit. However, the name is often associated with female dogs... If it were my daughter I would consider Amitza, which- while I have never heard used, has a nice ring.
In that sense, you could pioneer a new name for a boy, too, such as Amitz or Ometz (they probably sound close enough to common names like Amit and Omer).
If you need the Hebrew spelling to any of these, feel free to contact me directly.