*Long but please help* Am I just insecure like he says?

I’ve posted before. I dated someone I worked with for a year or so. My boyfriend pushed for this girl to be hired who ended up spreading rumors about me. My boyfriend always believed her over me when I would try to explain. To the point he threatened me into quitting. I forgave him because when I was talking to him after I felt like he was making sense. Maybe he manipulated me tho, idk. Anyways, she’s been working there since April. I quit in beginning of July. Through all that time, they BOTH swore to me that they didn’t talk, didn’t have each other added, whatever. Today I’m hanging out with him and he’s telling me that he’s talking to his boss. But I see Snapchat open so I’m definitely suspicious of him. So I question him. He ends up telling me that he had this girl on there. I was confused bc he claimed he was mad at her last time he said anything about her. So I was like “why?” And he was like “bc she added me” and I was like “don’t you think that’s weird? She has you on Facebook, snap, has your number (bc she’s manager too so they all have a group chat) But you claim she’s “just your sister’s friend”. Are you into her”. He swears he’s not. When I was pushing about when she added him he kept saying times that didn’t make sense and when I would call it he’d say “idk what to tell you then”. I asked him to delete her and said that would prove to me he’s not doing anything. He told me “you don’t need to be in my business” and keeps saying that I’m just “insecure over her and need to get over myself”. Like why would I be insecure over a girl who has some odd obsession with me? That’s her issue? MIND YOU this is the same guy who all of MY friends he automatically says are bad people, rapists, using me, etc. He keeps saying it’s not a big deal and keeps trying to turn the argument to be about something else. This girl has a boyfriend and a kid with him so I don’t think they’re doing anything. But I think he’s trying to make it seem like they’re not as close as they are for some reason. I tried to leave him a couple weeks ago and he basically begged me not to go. He also has a history of stalking one of his ex’s.