I don't know about object constancy, but I do know exactly the feeling that you mean. Connected and then... empty? nothing? And then feeling upset and disappointed about the emptiness. I have found that it is a little easier to access the positive feelings by myself as time goes on. Maybe like two and a half years ago, I would crave that connection with my T in a really intense way, but eventually it got less intense. I didn't have to fight off the emptiness because I somehow was able to offer those feelings to myself. I do still go through periods where I'm struggling more in general in my life and I want the warmth and comfort that my T provides, but when I'm doing okay, I find that the parts of her that I carry with me feel like enough.