I have ADHD and I don't recognise what you're saying about wanting to leave a relationship being a trait. Yes, relationships can be harder and yes, sometimes you can get a bit bored with life, but actually I think blaming you leaving your relationship on your ADHD is nonsense. If it was a good relationship and you were really in love with him and vice versa, you would have stayed. If it was down to ADHD you wouldn't have left him when you were doing the exciting thing of looking for a house together. Loads of people with ADHD are in long term, loving, maybe even slightly boring relationships.
But even if that's true. Even if you left him impulsively because you weren't getting enough dopamine, it doesn't mean it was a mistake.
He was/is a porn addict, remember, OP? The idea that your relationship was perfect while your then-partner was addictively using pornography is absolute nonsense. You may not have known about it at the time, but you would have been aware that sexually, things were weird, and your partner was being secretive and dishonest. You would have sensed that things weren't right and that you were not the centre of his world (because it was porn at the centre, because that's the nature of addiction.)
Please don't call him or write to him. You are using this idea of contact as a form of self harm. You are harming yourself with this idea that you ruined the perfect relationship. Your relationship was not perfect. It wasn't even all that good. One of you was bored and seeking more excitement and fun. The other was actively engaged in a harmful sexual addiction.
You have got to start facing up to reality.
Even if he wanted you back, you're not in a position to be in a relationship with anyone while you are so deep in denial, limerence and self-harm. While you are so desperately seeking validation from another person. Get yourself sorted. Find happiness on your own terms. Maybe you'll meet someone new and lovely, or maybe this person will come back around when the time is right. What's for you will not go by you, as they say. So focus on getting your mental health sorted before you look towards any relationship, whether it's this one or a new one.
As I say, I think the Crappy Childhood Fairy is a good resource. Also, Better Help is an online therapy service with reasonable prices, which might be a useful resource if you're waiting for some mental health support.