I lived with my exboyfriend for four months. The relationship turned abusive so I left eighteen days ago. I have furniture still in the home, and I do have receipts for it, plus I have mail that comes there as proof of residency. He changed the locks so can I forcefully let myself into the home(break a window) to get my belongings?

No, when you left, you voluntarily vacated. That gave him the right to change the locks. If you break in you will be committing a crime.So you need to make an appointment with him to get your things. If he refuses, see if the police will accompany you to retrieve your belongings. Otherwise you will have to go to court for an order to allow you to retrieve them.

Normally the police will go with you and allow you to get belongings.

if not, sue him in court for value of property. Lesson learned, if you move out, take your belongings