How to Win Him Back?

He and I have been together for almost 5 years and have a beautiful baby girl together. We even had marriage plans. Recently, there has been some mild tension and he suddenly decides he wants to end our relationship. Problem is, we live together and are staying together until the lease is up in about 4 months. I know the main reason it happened is because of my attitude, I tend to be very b*tchy but am making some great progress in eliminating that hostility from my life (a big problem was a boatload of issues with my parents, who I am distancing myself from to avoid further conflict). I have this odd feeling that maybe he just wants to go out and "test the waters" and would be ok with that if there is a good chance we'd get back together. We are both still in love with each other and still do everything together, ie. eat, sleep, play together, etc. and the only thing that has really changed is that he says we are no longer together. My time is running out fast! How do I win him back?!