The decision of whether to use first- or second-person depends on the type of writing.
A monologue is usually a dramatic piece of writing. From my experience, most monologues are always first-person. In a traditional monologue that an actor would use for an audition, the actor/character would be speaking from their own perspective and relating thoughts and emotions based on personal experience. To make the piece relatable and personal, the first-person "I" is recommended. The only reason I can think of for using the second-person "you" in this theatrical type of monologue is if the actor/character is talking to someone and attempting to explain events from the other person's point of view. For example,
Actor 1: "You would brush your hair, put on make-up, and act like nothing happened. You would leave the house for hours without a care about anyone else's feelings."
In this context, events are still being told from the actor's point of view and no one else's. Since the actor is the main character as opposed to the reader, this could still count as first person, but with the use of "you" it can also count as second person.
Getting outside of the theater, I believe it's perfectly fine to use both first and second person in a piece of writing that is similar to a personal essay or narrative. It depends on the tone you want your piece to have and whether the piece is informal or formal. Sometimes, if the piece is informal, you can get away with writing any way you want—as long as you have a distinct purpose for your writing and as long as that purpose isn't confusing to readers.