Aio? Please give me advice

Ok so long story short. Me and fiancee are 26, with a 1 year old and been together for 8 years. He propose to me last year but it defo felt rushed and force, I could feel he mainly did it as I kept talking about it. But anyway, we've grown with eachother in 8 years we've had such a good relationship with loads of ups and downs. I always trusted him, he was always so loyal to me. I know this because I've always been the type of girlfriend to look through his phone and stuff and vice versa. Last Christmas, just a few months of him proposing, I found texts of him and a new co-worker. They were super flirty and he called her names which really hurt me e.g. good morning beautiful and cutie. I read through the whole chat and it had been going on for around 2/3 weeks. The chat made it clear nothing physical every happened as in one of the messages she stated "nothing can or will ever happen between us as you still live with your girl and ur son" in which he agreed but said he really enjoyed talking to her and wanted to be friends. In 8 years of being with him he's never had a female friend so even that was a red flag but obvs it wasn't just friends if flirting is involved. I confronted him about the texts, he said he was sorry but felt so lost and we were going through a rough patch and she was making him "happy". He blamed it on me really, saying I was putting him down daily and being controlling which I have to be honest is true but I don't think that excuses his behaviour? After this confrontation we went a month of basically no contact but still living together. In the end we stayed together, we promised to both change and fight for eachother and make each other happy. It's been 8 months since, I still look through his phone because the trust is gone but she's blocked still and he is being loyal. He also changed jobs at the time as I asked him to. Same business but different location.

Now, his old boss is asking for him to start covering some shifts here and then in his old job where she works. I told him I don't feel comfortable with that, maybe as I don't really trust him. Idk. He said he understands but he misses working there and misses his friendships there with the boys. He said he'll cover shifts only if she's not working, he'll asks for the rota first. I agreed. He covered two shifts already and showed me she wasn't working and wasn't on the rota. Which is fine I guess. Now, this Sunday he's been put to work there again and I saw a copy of the rota and she's working. He hasn't told me about this yet. I'm not sure what he's planning. Is he just going to go and hide it from me? Is he gonna go and then tell me once he's there so I can't really say nothing?

Idk if I should ask him about this and tell him I don't feel comfortable in him going although he knows this. Or should I just trust him to be around her. What do I do? I really don't like the idea of him even looking at her or her looking at him. It makes me feel sick. Surely if he loved me he wouldn't take this shift. Aio?