Hi bb12,
There is even a line from a popular song lyrics, "You can get addicted to a certain kind of sadness
Like resignation to the end, always the end."
I think it is very common to be stuck in the feeling, because it feels familiar, even though it is not constructive. Also being happier and lighter after any kind of traumatic experience requires a lot of mindful activities and willpower to change the things. But it is OK if we take the road a bit slower.
I find myself sometimes having an opposite reaction. For example I wake up and feel sad a bit, then I think that it is probably because of him. To be honest I am over him. But more than for a year I was having the worst low moments in when I would wake up. It was my response to thoughts, rather than opposite as it happens now. I think it is more habitual, after having my brain trained for receiving that much stress and thinking non-stop about him, replaying the episodes, rethinking, constructing "what if" scenarios, now I find myself a bit confused

I do not have any strong feelings for him, I am almost fully detached in my rational mind (using some terminology), but it is harder to let emotions go, so I have the feeling of sadness or loneliness sometimes. But I am sure if we continue working - it will get better.
Good luck!