I can't afford to move out and continue to pay the mortgage so I need a financial agreement in place.
I wouldn't put it quite so bluntly - makes you seem needy or weak.
What about - "Let's sign a separation agreement so that I can move out and we both can move on with our lives". If he asks why you need the agreement to move out, just say "We need to have an agreement in place so that finances will be managed properly".
