The core answer of "you need a trust" was the one I hoped would be in the answer, and I am glad it was. I assume these parents know that no matter what the missing reasons might be, the relationship is unlikely to substantially change at this juncture. Leave them an inheritance via a trust (or don't, if you want to be a dick about it; you could leave it all to the Battersea Dogs' Home if you feel so inclined), and go the therapy or al-anon or something about your grief. That's all you can do.