Here "I" refers to Moses. The phrase "at that time" refers to when the Israelites were at Horeb, which is the same as Mount Sinai. Alternate translation: "When we were at Horeb, I spoke to you" (See: [[rc://en/ta/man/translate/figs-explicit]])
Here "to carry you" means "to lead you" or "to govern you." Alternate translation: "It is too much for me to lead you by myself" (See: [[rc://en/ta/man/translate/figs-idiom]])