i can't let go of whether he's an arsehole just to me. But that would seem unlikely.
You already know the answer to this. You said he cheated on his ex multiple times. Him being an arsehole is his problem, not yours. There's nothing you can do to change him.
You are addicted. Are you going to try some counselling to work out what's got you into this in the first place and how to overcome it? The thing that allowed me to go batshit over someone I didn't really respect, like or think was good-looking was exactly the same thing that led to me to act unfairly at times as the LO when I was younger. I personally didn't have counselling but worked it out for myself. But I imagine CBT could be helpful in this scenario.
You said it's action time...
Have you deleted his number out of your phone and email address out of your online address book, blocked on fb etc? All evidence of LO's existence needs to be deleted from your life. Like Self, you need to instigate ruthless NC now. Get a new phone number and do not give it to him. Give your home a massive spring clean/redecorate. Rearrange furniture and bin any item associated with LO.
All my sympathetic friends who listened to me overanalysing (the ones I didn't frustrate to the point that they avoided me) were really kind but it perpetuated the problem as it meant he still took up headspace. Ironically, the most helpful piece of advice I got was from the person for whom I'd been the unrequited LO years before! He went NC with me but we then managed to have a limited friendship years later, when he was happily married and, frankly, the scales had fallen from his eyes. He listened to me for a bit and then he just cut me short and told me "stop talking about him now. Do not ever mention him again." It was blunt. Rude. But bloody effective.
I was forced to think about other things in order to be able to hold a conversation for the rest of the evening. I socialised a lot after that so I was compelled to hold conversations about different topics. I stuck to the rule and eventually it became natural to think and talk about other things rather than forced. Go to exercise classes where someone is working you hard and shouting at you (spin or something), learn something new and tricky/technical (like a martial art maybe or climbing) so that for the duration of that activity you can't think about anything other than that activity, let alone the LO, plus it'll give you another source of feel good endorphins - one that you are in control of.
You will have to fake it until you make it but you will make it.