how come I feel so alone and abandoned even though I live...

I sometimes find myself feeling anxious when I'm home alone, but I try to find things that I love to do that my partner doesn't necessarily. I'll learn a new song on the guitar, take a bubble bath, make a big salad, etc. Finding things to do that help you connect with yourself was a good start for me to show some self love and care when feeling lonely.

Separation Anxiety? Fear of Abandonment? these are caused by trauma. See a therapist if you aren't already, and try to see if you can work on self-care when you are alone. Usually a discomfort when alone can stem from co-dependency or lack of self worth, so be sure you're working on taking good care of yourself both mentally and physically.