I am trying to find a story. The MC time travels to his body as a youth. A detail I remember is the analogy of time as a river is used. Any attempt to change events from the previous timeline being resisted. The resistance is likened to a ripple in the river being smoothed over. The MC tries to save his sister from a fatal car accident but time tries to take her using another car accident, which becomes the reason we learn it is hard to change the timeline's events.