Worried about my BF’s relationship with his work wife. How can we move forward with trust?

My boyfriend and I have been seeing each other for a year now. We break up for two weeks and strong as ever the last 4 months.

However my boyfriend was frequently discussing his female friend who helped him land a job.

I asked if they ever had a thing, he said he used to have a crush on her when they first met 5 years ago, they hooked up once and have remained friends ever since. One day he showed me a video the girl sent him. It was the girls mom saying that she should go for him and she wrote DTF?

That alarmed me but he said he didn’t have feelings for her. I couldn’t stop thinking about that video and how frequent they text. So I decided to look at his phone while he was sleeping.

I discovered sexual texts from him to her saying he wanting to sleep with her the day after we break up.

I also found texts from her to him while we WERE together saying “break up with her and date me” and “ I want to kiss you” “I’m prettier than her”. He was sending her the same pictures he would send me, and sometimes calling her pet names similar to what he would say to me. They would make plans to hang out but never do.

I confronted him about it. He said I can’t be mad because we were broken up at the time. He said he doesn’t have feelings for her and she has a boyfriend now. I asked him why didn’t he cut her off when she confessed her feelings to him, he said he didn’t want to upset her.

He insists they remain friends because she did him a favour by getting him this job. I told him I don’t feel comfortable if they hang out alone. He said he wouldn’t but it’s almost impossible since they work together now and go to lunch.

He said I was mentally ill for invading his privacy, said I’m the one who has issues, that I create my own misery and paranoia, that I make up things in my head that aren’t true.

He is upset with me for these accusations and has been distant. He said I need to get over it or leave. He says the only thing that is pulling us a part is me. How can I gain trust again?

Updates
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Update: I confronted him again and he said he only wants me. He got defensive because he finds it “glaringly” obvious that I’m the one. He said this girl has been his friend for years. She let him and his friend stay with her and her boyfriend at the time, she helped him get this job which practically changed his life. He apologized for not being more stern with her and will shut it down right away if it happens again. He feels aweful if he has to block her because she was there for him.
Worried about my BF’s relationship with his work wife. How can we move forward with trust?
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