But he's had no practice in honesty until he met you.
So you are right to be wary of being dumped abruptly again. In my opinon, you're involved with a liar, who could be a sociopath. They are masters at pretending to feel normal feelings of love and caring, but it's all an act. Looking right into your eyes and lying to your face, so sweetly and convincingly? That's what they do.
RED FLAG! You heard multiple stories of his lying and cheating, and when you discussed it with Peter, you were not comforted. You were still alarmed.
Keeping a journal about his lying and cheating, even if he claims to be self hating to have done it, is a way out of his boredom. Narcissists are easily bored. They have a deep abyss of nothingness inside. They USE people to allay their boredom, idealising a new lover, until they get bored with her too. Then she is demoted and discarded for a new model. He will find some other new chick in your biking community. My fear is he uses the community as a habitat for new prey.
He is "comfortable" lying, he is selfish, he plays people. Why do you not see, he is probably playing you now? Others have tried to warn you. You're taken in by his ability to act normal. It's an act. He can't love.
So he says. It's a red flag that you're only 9 months into this, and have had MANY conversations about his former lies and cheating. Of course he's laid himself bare. He probably gets a thrill out of telling you how badly he treated others, and yet there you are, a sitting duck, trying to trust him.
Read up on narcissists. They are invariable charming, often good looking, sexy as hell, etc., etc. They start out being so attentive. Chatting online or texting a lot. Nice dates. Flowers. Compliments. You feel like you've found the perfect man. Until he gets bored. This can take a varying amount of time, usually it's when he feels he's well and truly hooked you. I don't think he feels he has yet, so he's "going to therapy" to further gain your trust. I question whether he really is going...
Your experience of him as a great person is just too much in conflict with what you've heard about his immediate past. Just before you he was a lying slut. Now suddenly he's going to be all honest and true? Hmph.
Go ahead and tell him you looked at his journal. He might've left it lying around as bait anyway, to make you seem the bad guy so he can use that against you when the lying and gaslighting and fights begin.
Please check out this website and forum.
https://www.psychopathfree.com/
That's the main page. Go read at the forum and see hundreds of stories like your own.
Read the articles here
https://www.psychopathfree.com/articles/
Which tell of warning signs and red flags about how to spot a narcissist.
Here is a sample article.
https://www.psychopathfree.com/articles/why-sociopaths-idealize-and-devalue-people.370/
Top 7 ways to spot a narcissist
https://www.psychopathfree.com/articles/top-7-ways-to-spot-a-sociopath-psychopath-or-narcissist.342/
10 warning signs you're dealing with a narcissist
https://www.psychopathfree.com/arti...ith-a-sociopath-narcissist-or-psychopath.340/
30 red flags you're with a narc
https://www.psychopathfree.com/arti...ith-a-sociopath-narcissist-or-psychopath.340/
Unlike here, it is geared towards the type of person your SO seems to be. If I seem so sure he is a sociopath, it is because I was with one for 2 1/2 years... I have learned to look out for red flags. I was so hurt by how I was treated. They can seem like the perfect person, until suddenly they start to pull away. My guy was a self described poly guy, but it didn't prevent him from unethical behavior and a lack of real humanity.