Bella doesn't ever think she is pretty
Bella didn't grow up pretty.
Bella's not particularly charismatic or intelligent - only sort of. Bella never received much attention, until she started to grow into her looks. Now, like some sort of junkie, Bella's greedy soul (aren't we all) wants the nourishment that comes from adoration (don't we all?). This kind of behaviour and the ensuing result may be rotting her from the inside out, but she believes it prevents her from being swept away by the not-caring everywhere else.
Bella doesn't know what specific kind of care her attention-seeking brings; she's not a man, and she probably knows less about men than she lets on. All she knows is that these actions bring about a swift and pleasant reaction. She doesn't understand why no one stays.
Bella, like you, just wants to be loved.