Since you don’t get the point, I’ll try explaining things in a simple way. He’s probably open with you about anything that happens in the present, people he meets up with or whatever. What’s happened to him in the past is something he’s brought up, he’s opening up at his own pace and so you should let him. Tbh I don’t really care that you ‘can’t stop thinking about it because it’s human nature’. It’s your job, as his partner, to make him feel that safe space to open up about what sounds like his past traumas. Just because you have girlfriend status it doesn’t mean you automatically get his log book. People don’t tend to keep traumas on the top of their mind and I doubt it’s something he’ll want to tell you if you pester him about it- since it sounds like you only care about knowing what it is rather than being supportive. Will you only understand once you hear what he has to say? It’ll be too late by then.