You are confusing the determination with positivity.
The first statement, is in fact negative, it expects difficulty, perhaps what would normally be considered insurmountable difficulties. It expresses determination to overcome any such difficulty, but that very expression of determination is an oblique warning of the possibility of failure in face of the expected problems.
The second statement, is positive, it doesn’t expect any difficulty, and if difficulties occur, expect’s them to be easily dealt with. Only truly unforeseeable and insurmountable problems could prevent a successful conclusion. Such as God personally deciding that it won’t happen.
You would not say “I will get a gallon of milk from the store, come hell or high water” except as a joke, there’s no reasonable difficulty where that would be appropriate. You could say “ I will get a gallon of milk from the store, god willing and the creek don’t rise” without joking.
You would not say to someone fighting cancer, “You can beat this, God willing and the creek don’t rise”, it’s too positive about the outcome (not to mention that it may feel like God already has it in for them). It brushes the expected difficulty aside as if the task ahead was no more difficult than getting a gallon of milk from the store. You absolutely could tell someone fighting cancer, “I know you can beat this, come hell or high water”. You expect a good approximation of hell to happen, but know that they will give everything they got to get through it.