Rental application reason for leaving - I've been paying my ex rent while living with him

I've been living with my boyfriend for about a year and a half, we dont have an official agreement but I send him money every month to help out and we treat it like rent. We recently broke up and I'm now apartment searching. We're leaving on decent terms though and he's agreed to vouch for me as a good "tenant" (Im not sure if that's actually what I would be consider here).

I'm filling out a rental application and it asks for the reason for leaving my current place. I don't want to give too much information but I can't think of a professional sounding way to explain.