When people tell you to forget the past, especially family, it's not always wholly representative of their looking out for us, but has more to do with our memories being inconvenient to them, interruptive to ways in which they'd prefer to think of the past. We're all motivated to frame things in somewhat different ways.

Personally, I've found that most people's ability to gauge whether others are actually "dwelling" on the past tends to be grossly inaccurate. I'd tell them that unless they're prepared to present precise proof that you're allowing significant events of the past to live in the present in corruptive ways, they should go kick rocks. But that's just me.

Certainly you don't want every conversation to lead back to sad events of the past, but the people that most care about you will understand that it comes up now and again, and will be honored by your trust in them. The rest mostly just don't know any better.