I also recall Mark commenting about Di's drug use and she's also said herself that she used lot of drugs in her youth. I think she went off travelling a lot and would leave Mark with her parents, I remember he said he would wait by a window for her to show up for events such as his birthday and it was always pot luck if she'd show. Viewing the vlogs it seems she's really tried to build on their relationship but obviously Mark still has a lot of barriers he probably won't ever be able to get over