D&C 54

 The Lord takes it very serious when we make covenants with Him, for when they are broken (of course by us), He stated - 

"...as the covenant which they made unto me has been broken, even so it has become void and of none effect.

5 And wo to him by whom this offense cometh, for it had been better for him that he had been drowned in the depth of the sea."  (Doctrine and Covenants 54:4–5)

In other words, all promises are lost.