My boyfriend can't accept my past

A little back story: I went through a tough time right before meeting my boyfriend that really changed who I was as a person for a little while. A week before I met my boyfriend I had a one night stand with a guy I had met once. As somebody who's only ever kissed 8 people and had sex with one person prior to that (a long term boyfriend) it was very out of character and something I regretted immediately. It was what made me want to go back to who I was before. When I met my boyfriend, it made me feel like I could be. I felt myself around him and he put me at ease. I knew I loved him from the moment I met him, and out of fear of giving him the wrong impression of me, I told him the one night stand had happened a month or so prior. Rather than a week. I still gave him the full truth about it happening and I explained the hardships I went through at that time and he understood. About a month into our relationship he went through my phone and found messages to my friends from the night of the one night stand, and figured out when it actually happened, and that I had lied to him. It's been a few months now and he cannot get past the fact it happened. He's forgiven the lying, and he feels that he's getting his trust in me back. But he's haunted by the fact I slept with somebody before I met him, once telling me it made some things between him and I meaningless, he can't be in my home because all he thinks about is how it happened there, he daily gets upset thinking about it, anything sexual in TV or movies reminds him of it, he just can't accept the fact I slept with somebody in a meaningless context. I don't believe I am in the right in the slightest, as I did betray his trust and lied, something he told me he was very sensitive to when I first met him. But it makes me feel like such a villain for something I'm already so ashamed of. Something I couldn't have controlled the timing of, as I hadn't met him until after it happened. He believes he has some form of bipolar disorders or something along those lines but refuses to get help from professionals about it. And doesn't talk to anybody about it apart from me. I feel so helpless and lost, he doesn't want to leave me and he doesn't want me to leave him. I don't want to leave him, but I feel like he's just hurting so much because of it and I'm powerless to do anything about it. I love him and I want him to be happy and he insists he is happy but I just feel like if he won't take help this is never going to recover. What can I do? How do I help him? How can I coax him into accepting help? And what if he doesn't take it? I just don't know what to do anymore.