"Stuck in a rut" is an idiom that means being mired in routine. The figurative phrase calls to mind the image of a wagon wheel hopelessly trapped in a deep rut on an unpaved road.
The image of a stuck wagon wheel suggests the origin of the saying. Becoming trapped in quicksand or a quagmire conveys the same image. Here is an example of the phrase used in a sentence: "Spending 17 years as a postal clerk and as many years in a boring marriage made Sam feel stuck in a rut." Sometimes this sense of being stuck leads to a breakthrough and the beginning of progress; other times, it just leads to a feeling of hopelessness and helplessness.