OP I've changed my username for this, because I think you need to hear this, and I mean this in the kindest possible way. I know you are pregnant and I don't want to cause you distress, but I feel like what others are saying isn't really getting through to you.
I was in a 'relationship' with a man who had a long term partner/kids for 18 months. He promsied me he'd leave after we were 'dating' for just 4 weeks. He obviously didn't. I ended it after around 15 months as I was sick and tired of waiting for him and him bottling it every time he said he'd leave. He told her the truth, she obviously was extremely angry and rightly so. He left, moved to his mums, but couldn't cope with being separated from his children (or likely her too, I don't know and it doesn't matter.) So he went back to her pretty much immediately.
He has never not messaged me for a single day since. He stayed with her, for a further 6 weeks. Every single day he messaged me saying he's made a mistake but he's terrified of losing his children. He never had a bad word to say about his partner, in his words they had just grown apart. He had love for her, but he says as the mother of his children. He would tell me he loved me, wanted me, the same usual spiel they always come out with.
Anyway - he did leave again - and I do know it was his decision but his partner actually messaged me but that's not part of this story. He left her (the second time) and has been separated since. For the last 6 months he has tried to be with me.
I'm not saying this in a 'I won' way, nobody won in this situation, and I don't know what the future holds. What I am telling you, however, is that if there were genuine feelings there, they don't just turn off. Your partner may be with you, like mine did, he went back to his partner, but he couldn't keep the pretence going long term.
I know it must be so incredibly hard, but he has shown you over and over he isn't prioritising you. If he wanted to be with you, he would have ended it with her and not had any contact with her since. He'd have respected you so much that he wouldn't redownload instagram and look at her, because his love for you and remorse at what he'd done would trump that.
I wish you the very best, but these wise MN ladies have been where you have and they're offering advice as they were once you, and they wished somebody had told them at the time what they're advising you. Listen to them, really listen, because once you have that realisation that you and your baby deserve more, your new life will truly begin. Take care.