What is the difference between guide and instruct?

To "guide" is to lead the way. You teach by showing where the next best steps to the goal are, thus avoiding missteps and dead-ends by the one you are guiding. The one you are teaching gets there by following your guidance.

To "instruct" is to instill information into the one you are teaching. The one you are teaching learns by being given the relevant information needed to reach the goal , rather than by being led step-by-step to the solution by a guide.

Guidance is a form of instruction, but not all instruction involves step-by-step guidance.