It sounds like you need to establish a boundary right up front. Because otherwise you’re going to be living with this for the duration.

’‘ I appreciate that you need to vent, but I’ve got my own things going on, and this is too much. “ You don’t have to be mean about it, but you do have to say something.

This person is taking advantage of your politeness. You don’t owe them an explanation. Just tell them you don’t have the bandwidth to be their emotional support person.