will not gloat over me

"will not delight over my trouble"

If my foot slips

Here "my foot" represents the writer. The slipping of his foot is a metaphor that refers to the writer's troubles and misfortunes. AT: "If I make mistakes that cause me trouble" (See: rc://en/ta/man/translate/figs-synecdoche and rc://en/ta/man/translate/figs-metaphor)