I don’t how or if I should trust him (very long)

My boyfriend and I have been together for 10 months and I don’t trust him, I used to but then a girl from his work (we shall call D) has been all over him and now I’m not sure.

It started 2 months this into our relationship where he had told me D had messaged him one night while I was at working saying she’s horny and wants to meet him, he said no because he had a gf (apparently) and she then started talking about her sex life. Ever since then I’ve hated her and he knows this.

She then messaged him while I was at his asking for a lift home when she was drunk, I begged him to delete her off snapchat and he put up a fight and refused to because he didn’t want things to be awkward at work even though he knew I felt so low about it. He eventually deleted he but made his setting so anyone could message him.

Fast forwarding to this weekend D invited him to her house party where he didn’t go, I told him I don’t trust her and he got defensive saying how “she’s not a bad person and it’s not like that anymore yeah they had a conversation about something they shouldn’t of had but it was a long time ago” and now all of my trust in him has gone.

He knew from the start I found it hard to trust people and the fact he was talking about sex with her in our honey moon phase shows he wasn’t as obsessed with me as I was with him.

Shall I speak to him and try and work out a way I can trust him again or shall I message D to try and find the truth? There’s also little things like he hides his phone from me and he also deleted a conversation between him and her once, no idea what it was about tho 😕😕