Cheating partner

So long story here, I’ve been with my partner going on 4 years next month. Our son will be turning 2 this year. I found out maybe like 2 weeks ago that he’s been talking to someone for 8 months. I texted her & told her to back off. She said they were in a serious relationship & I’m like wtf?? Like what could be more serious than buying a home together, having a child together ?? He kept telling me they were friends & she finally sent me screenshots were he tells her he loves her & calls her baby & stuff like that. BUT me & him still live together, sleep in the same bed, have sex & basically just everything together. I’ve been a stay at home mom since our son was born & idk what to do. She said she wants to be with him & I’m like girl I just told you the truth how do you still want to be with him?? Like do I just continue being with him knowing that he’s with her while I can figure something out financially for myself ?? I’m fully dependent of him & it makes it hard for me to leave or kick him out or do anything. Also, he saw this guy sent me a friend request & now he’s talking shit I’m just so confused because why when you have a whole other relationship! I feel like he’s just playing both of us at this point. He tells her we live together for our son & denies that we sleep in the same bed & that we have sex. & I actually caught him on video admitting that we do all that stuff & sent it to her & she’s still there !! I don’t get why. I don’t get why I’m still here either LOL. Like this girl believed him when he told her he was “single asf” & I was like why would you believe that when we live together, you guys only talk when he’s not around me, & you’ve never actually hung out so how could you continue with someone like that. I’m not just blaming her, I blame him too for pursuing her

There is so much to this story but I just posted the main details I guess.