My life was like yours. Living together as a nuclear family; shared home/furniture/kitchen stuff; kid in elementary school (hers from a prior relationship, but essentially my stepson); shared money.
My WxGF dumped me for the AP. Initially she told me she would be moving out of our shared home and in with him, so I waited. About a month of pure Hell. Coming home on a Friday or Saturday night to find them canoodling in our living room. He became almost another roommate. They set up a bower on the floor of the son's room, leaving me alone in our (previously shared) bedroom.
Eventually, I grew impatient waiting for her to move. Among other shared things, we shared a car - a small subcompact. We each also had a motorcycle which was my main transportation. I bought myself a used pickup truck -- a small Toyota with a shell on the back. Found a bedroom for sublease in a 2-br house. The main tenant was a single man like me, fresh out of a broken relationship. Packed up my clothes and my guitars and amps and moved in with him. I also took my bicycle and bike trainer.
Other than my clothes, guitars, and bicycle, and separating bank accounts, I left everything behind. Furniture, stereo, television, kitchen stuff. I bought a new stereo (still have that -- it's a nice one -- Dolby surround sound media receiver) and a television (been replaced multiple times since) for the shared living room with my new roommate. Two single guys with busy careers, learning to date, we almost never cooked. Our fridge might have a few beers, a few bottles of wine or champagne, some mustard. Maybe some rotting Chinese leftovers.
As to the son, he was attending elementary school so he stayed with his mother in their home during the week. I would see him on weekends, and he would stay with me when she traveled for work (which was frequent). In addition to the usual domestic stuff, we'd go to sports games, movies, or concerts. I'd take him to his weekend soccer matches and such.
We're still close to this day. He's now in his late 30's. I married my current wife when he was a senior in high school. He was the best man in my wedding. He's also the Godfather of my first biological child.