# I spoke to you at that time

"When we were at Horeb, I spoke to you". 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. 

# I am not able to carry you myself alone

"It is too much for me to lead you by myself". Here "to carry you" means "to lead you" or "to govern you."