I relate to this a lot. Some of the times I have felt the most at peace and confident with myself are when I was receiving lots of attention from different guys while single and I was too busy to feel the all too familiar feelings of confusion, emptiness, and unworthiness that come with what I attribute to CEN. I know my partner loves me and I love him for who he is - but I still miss getting that amount of attention. It's the attention that I always wish I'd gotten from my parents and other family members. Feeling like I'm actually being seen for who I am.
I agree that focusing on my own personal hobbies and fitness has helped out a lot. Even if I'm not single, I still want to feel attractive in terms of my physical body and my interests - just to keep the jealousy of others at bay, and also so that I don't have regrets about what I could have accomplished. I think it helps although the loneliness comes and goes.