Please help on this..

Okay so me and my fiance have been together for 3 years now. He was married before back in 2012 they was only together for 2 years. His ex wife had 4 kids. He helped her raise them while they were together. They arent his kids. I am pregnant with our son and am 33 weeks now. Well off and on our relationship i have caught him talking to his ex wife, nothing major just mainly talking at least Thats all ive seen. He has cheated on me before a long time ago when we first started dating. Well he always still to this day talks about her kids and stuff and barely talks about our son. Ive mentioned it to him before that it bothers me yet he continues to do so. He says theres nothing going on between him and his ex wife and that hes happy with me. It hurts knowing he says things like he misses her kids and stuff so i got her number from his phone cause she had kept on texting one night and his phone kept going off so i checked it. Well anyways i was nice And everything to her when i wanted to cuss her out and stuff but i basically told her that i dont appreciate them talking cause it hurts and knowing that i feel like if he had the chance to hed run to her. Seriously thats how much he seems to miss her. He talks about her a lot and everything. But anyways im friends with her little brothers girlfriend on Facebook and she posted a thing about a really messy diaper ill add the picture but he commented on it saying theres not a day goes by that he dont miss them. It bothers me i honestly wanna tell him i dont think were gonna work out anymore because i feel like he wants to be with her and her kids. I literally just wanna cry. Im so fed up with this.