So, I caught my boyfriend in a lie. But the way I found out it was a lie... is bad. I read through his journal, only really looking for things about me. On the 15th of July 2019 (we met May of 2019) he wrote that he was sexual with his friend. He also wrote that day that he was afraid he didn’t like me, but then crossed it out saying he liked me because I fucked him. He asked me to be his girlfriend two weeks later in late July. I have been jealous of this friend before, and told him earlier this week that I was fixating on that time period early in our relationship when he would hang out with this certain friend (who he has since stopped talking to because I was jealous, never asked him to or anything, he just kinda stopped because he knew it hurt my feelings) and basically that I was afraid he had cheated on me with her. And to this, he told me he had not been sexually active with her since 9 months before he and I met. Which I know is a lie!!!! He actually said something like “I’m wracking my brain trying to think if we were, and I don’t think were” 
We have come so far since then, this happened last summer! Since then we have moved in together and I thought we were open communicators about our sexual history and our current sexual needs. I’m not worried he’d cheat on me now. I’m just so upset he has lied to me about the last time he was sexual with this friend, and I’m worried it may have continued after he asked me to be his girlfriend. And that the guilt drove him to stop talking to her, since I really did try to play it cool over their friendship. And the only reason I know he is lying is because I violated his privacy. I don’t know what to do. If I should even do anything, or just forget about it? Our relationship has grown a lot since then, but I also feel slightly taken advantage of and don’t want to be the fool.