My mom and I are stuck on a lease because my boyfriend left

My (25F) boyfriend (28M) 1 year relarionship. just got into a huge argument over him still being super close with his ex girlfriend of 3 years that he used to live with and not being transparent about it. He said they broke up 3 years ago but I found out they were together 1 month before me. Also, he told me she was his cousin then confessed before asking me to be his girlfriend. They were still living together at this time. She’s super close with his family so I trusted him, because she is, I don’t know her. But i found out they were seeing eachother a month before me when he said it’s been three years. I brought it up to him and he said that I crossed a line. How funny. My mother makes decent money and has a good credit score. She signed the lease for us so we can live together and it has been going well for around 9 months. Me and my mom are on the lease, he isnt because he isn’t American and doesn’t have any credit. After I brought up that lie, I “crossed a line” and he packed up his two bags out of my fully furnished $1,600 apartment. He’s gone. I’m stuck with the lease, with my mom, and we can’t afford it. I don’t know how to move forward from this. And me and my mom need help. Any advice? He told me he wanted to propose to me soon, we talked about our future kids, honeymoon, and wedding.

tl;dr
My mom and I are now stuck with a lease we signed because my boyfriend left me and we don’t know how to recover from this.