He’s my first, I’m his millionth

Me and my fiancé have been together 4 years, since I was 14 and he was 16. We’ve been engaged 2 years and had our first baby 3 months ago. Been living together for nearly 2 years. Last night we were just talking about random stuff and got on the subject of cheating. Ive obviously never been cheated on or cheated because hes my first relationship but he’s been cheated on before. And he turned round and said, he’s nearly 100% sure I’m going to cheat on him one day, maybe it could be from past experiences but they were technically kids when he’s had other girlfriends, we are adults now, parents. And when I asked if he even trusts me he said he doesn’t know. When this subject came up before a couple years ago, none of this was mentioned and he said he trusts me 100%. I don’t know what’s changed, we are still having just as much sex as we were before the baby nothings changed there. Yeah maybe I snap a little more as he doesn’t help as much as he should but that’s about the only thing that’s changed. This information comes after I found out he’s gambling. First our money, the baby’s money, only little bits here and there but it adds up, and today I found out that he’s stole my mums bank details to try and use her bank but it got declined because it was empty. But it’s the fact he tried. And even thought about it. He’s also started messaging a girl he used to go to a club with when he was younger and she put on her Snapchat story screenshot to get close and I’ll make the effort! and he screenshotted it? Deleted it but forgot to delete it out of his deleted folder. And they have been talking ever since nearly everyday. And obviously I can’t see the messages as they aren’t saved. I’m so confused. Do you think he’s decided to say I’ll cheat because he could be? Or could end up cheating on me? I don’t know what to do. I’m thinking I may leave it a week and if they are still taking I’ll have to talk to him about it see what they are ‘getting close’ about. I trust 100% but this just just seems suspicious and I don’t think I should trust as much as I would like too. The past 4 years have been great and this has only just started in the last couple weeks.