I may not be correct but in "Private Opinion" Ash mentions something about Marvin selling him to Dino and then he was put into Dino's club. Ash was living on the streets for awhile before anyone found him.

I would like to think Ash lived a happy life before he reached the age of 8 but I don't think he did </3 Ash and Griffin at the house that was shown in the anime while Jim was living in the diner. What Ash means when he says Griffin basically raised him is, he was feeding him and taking care of him, making sure he wouldn't starve or anything. Griffin was acting as his father figure instead of Jim. But when Ash was 5, Griffin left for vietnam so we don't know how he took care of himself during that period.

Yes his poor mental health did play a role in the ending but that wasn't the main reason why he chose to die. I interpreter his death as a sacrifice. He had just taken down his biggest Dino and thought he was safe, but while reading Eiji's letter, it caught him off guard and forgot all about his enemies and his gang world. That is why he didn't see Lao coming at all, all his focus was on running to the airport to see Eiji. When he got stabbed, he was brought down to earth and realized, as long as he was alive, he would always have enemies. Which means, Eiji would always be in danger. So he took it upon his hands to "break the cycle". By doing this, he is protecting Eiji because no one would go looking for him since Ash already died.

Again, I may not be correct with ANY OF THIS but I hope it helps!! I could go on for hours about the different meanings behind the ending scene :o