Thanks for playing!
For the check system, there should usually be a notification in the top left whenever you pass or fail, with a few exceptions (getting chocolate on valentine's day, or checking if you have an ability from temperance training). Failing some checks is normal, you'll never enough stats to pass everything.
All fortitude (or any other stat) could work, at least up until reaching the cap, you'll just have to pick the appropriate option for your stat during combat.