If you're slacking too much, you haven't found what interests you yet. Keep at it, but keep enough cards to palm off to society to buy your time and space.
Intelligence is variable. In my opinion, a strong life-long curiosity of a specific kind is what characterizes INTPs.
Relationships: a compatibility issue. It can be analyzed ad infinitum, but the point is that the people who are right for us are also rare, so probability works against us. Try again, and know what you are looking for. The latter is much harder than it sounds.
I don't consider myself part of a group, and I don't know how you define "success". That said, I'm very happy to be me. There's no reason to spend all your time pondering life's little mysteries if you don't find it worthwhile.
This is a page that cites statistics, mostly for Edahn's benefit.