Why not just be honest? Losing my mind!!!

So my partner and I split for a bit and he began seeing someone in that time we decided to make a go of things again this year in January. However I had several trust issues due to his lies/cheating in the past. He came into this telling me doesn’t have social media and has deactivated everything, like fine whatever. But as it turns out last night I saw his got an Instagram account, he also has Snapchat which shows his been active recently so I’ve confronted him about this and he tried to tell me he doesn’t. So I sent him the screenshot of his account and he denied it being his account and he said he could show me he doesn’t have the insta app and his account is deactivated etc and that’s not him. I got annoyed and eventually he stopped denying and started saying “as far as I was concerned I thought it was deactivated” he won’t address Snapchat and says he doesn’t use it but when searched shows his been active. Now my issue here isn’t the socials it’s the denial and the lies. Why lie? Even when confronted why say it’s not yours? His now saying I’m being ridiculous and trying to say stuff like “if this is what’s important to you then that’s ridiculous, I am just trying to get married, why can’t you love me, give me benefit of the doubt” etc but I’m trying to explain to him his literally denied the account being his when confronted at first like my point is why hide it or lie if you got nothing to lie about? But his twisting it all on to me like I’m the problem and I just want our relationship to end and this is an excuse. I feel like I’m going crazy!