Simply, “lo yarpecha” means that HaKadosh Baruch Hu will not weaken His guarding of us. But Rashi explains: “Hashem will not give you [cause for] weakness [resulting] from your being forsaken by Him.” The Minchas Elazar explains the meaning of this:
When weakness and despair spread through the heart of a person, he actively brings upon himself abandonment. It can be compared to a sick person whose doctor diagnoses him with a severe and terminal disease, and the patient may deteriorate simply from the despair that envelops him as a result, even if it turns out in the end that the doctor was mistaken in his diagnosis and his disease is curable.
This is what the passuk is saying: Not only will HaKadosh Baruch Hu be with you, and will not leave you chalilah, but He will not allow you to reach the state of weakness that will cause you to harm yourself, actively and “be forsaken by Him.”
Imrei Davash [Rav D.B. Schwartz] p. 149
