[on a sheet of paper folded in half and left on the table where he'll see it immediately:]
This is not your fault.
I've reached a point where I need time alone. I need to be living alone; I've never done that before I came here, and I only had a month or two before you arrived. Not giving myself this time was just making me angry and depressed, and that's not good. It was only a matter of time before I did something stupid with you.
I'm sorry to do it this way, but it's easier for both of us to have some time alone to cool down before we talk about it.