The first thing that comes to mind would be indeed obsession/ paranoia, but it's too broad of a term.
The second one would be the critical inner voice that keeps reminding us that we aren’t good enough and don’t deserve love etc. And the other terms in this category like critical- or negative-self image and negative self-talk.
Also, this negative attitude makes me think of a number of emotions and feelings like that
I'm not good enough to be liked; I'm boring/ ugly/ stupid; no one wants to listen to me; no one cares about me; I have no one; I’m alone and these insecurities might be expressed by these words:
I'm not good enough: insecurity < low self-esteem < inferiority complex < self-hatred
I'm alone: distant < seclusion < loneliness < social isolation (the most extreme)
As you can see this is quite a list and at some point meanings of these words may slightly overlap, so see for yourself if any of these words 'hits you' as the one you were looking for.