What do I do, is partner still in love with his ex, though he says he never loved her?

So my partner and I have been together almost 5 years. He is step dad to my son and we have 2 girls of our own. Over the period of time we have been together he constantly runs back to his ex when things get bad and trash talks me and if she contacts him sulking about her life he always enables her by replying to everything. He always hides it and lies about it and comes up with bullshit excuses. Everytime it happens we talk it out and he promises to block her and never contact her again and blah blah blah. Over all this obviously my trust has begun to wear thin. Recently he left his phone on the bench and early in the morning up popped a Facebook message on the screen from this ex saying “hey baby what’s up”. I couldn’t stop myself and picked it up and opened the message. He has a strong history of cheating and lying. Anyways so I scrolled back to the top and read all the messages. It was message after message of him trash talking me to his ex. then I get further down and he’s telling her he cheated on me with a work mate.then it’s back to trash talking me literally for a month straight of trash talk about me.I’ve confronted him and for the first time I think he has taken me seriously but I’m still having doubts and trust issues.I’ve given him another chance but with boundaries but I just don’t know how I can ever build back that trust with him and I just don’t even know if I feel the same way I used to. There’s no defined proof either way as to whether he did cheat on me with his work mate but she said they didn’t and he says he didn’t.