Telling you about difficult experiences that make him seem sweet and vulnerable can be a manipulation tactic rather than a cry for help. I'm not saying that's definitely the case here, but it is a possibility. Sharing traumatic/personal information very early on this can be a red flag for this. How long have you been together?
I don't think it's right that ADHD doesn't make people say this if you don't experience ADHD yourself how do you know they don't say this? And it's not just ADHD he's suffered with other mental health issues and had low periods throughout his childhood and things surely him opening up telling me how he feels is like a cry for help? He could have chosen not to talk to me and just blocked me without any explanation but he didn't
I have ADHD. I am telling you that ADHD does not make people say this.
I have also suffered with significant mental health issues as the result of an abusive childhood.
People with ADHD, mental health problems and adverse childhood experiences CAN understand and care about how their behaviour affects others; if he can’t (or won’t) none of these things are an excuse.
He’s explicitly told you he’s no good for you, you can do better, and that he will let you down. He is backing these statements up with actions, showing you that cares about himself and his needs, but doesn’t give much thought to your feelings.
It is not your job to save him. From someone who has ended up in relationships with ‘lovely but vulnerable’ men who declared their past traumas/mental health issues early on then quickly transformed into selfish pricks who only cared about themselves (while telling me I was the only one who had ever understood them, which definitely kept me hooked) - I’d say get out while you can. I understand the urge to help, but he needs to help himself first.
His problems and how he handles them are already affecting you. He should care about your emotional and mental wellbeing as much as you care about his.
He might need help to overcome these issues, but that needs to be from a therapist, not from you.