I don't want to be alone anymore...

A very lonely whumpee.

Maybe they have some power/trait that others fear, and are cast away or mistreated because of it.

Maybe someone on their life repeated over and over again that they are not worthy of friendship/love.

Maybe they simply doesn't know how to be close to other people.

Animes
Akatsuki no Yona


 * This is one of the main traits of Shin-ah, having lived in isolation and feared by the villagers, his whole life. He had a dark and empty cave for a room by the time Yona found him, and he didn't know how to react in face of her offered hand. Once part of the group, Shin-ah doesn't know how to act around the others, and rarely speaks. He didn't even had a name before leaving the cave, it's Yona who gives him one.
 * Although full of smiles and silly behavior, Zeno is a pretty lonely person. Immortal and from the very first generation (on his case, the only one) of dragons, he spend centuries wandering alone, unable to die.

Ao no Exorcist


 * Rin spends most of the second season (practically the first half) very lonely, as all his friends found out about his father and are either angry or scared of him.

TV Shows
Dirk Gently's Holistic Detective Agency


 * All 5 main characters starts as very lonely people, for different reasons. Dirk Gently (Samuel Barnett), for his abilities and history of abuse. Bart Curlish (Fiona Dourif), as she never really had any contact with any other person (unless to kill them). Amanda Brotzman (Hannah Marks), for her condition, living isolated from the world; Todd Brotzman (Elijah Wood), with the weight of all his lies and self-pity. And finally, Farah Black (Jade Eshete), feeling different, and had not encountering someone who understands her.

Grey's Anatomy


 * Alex Karev (Justin Chambers), although doesn't showing much, is pretty lonely on the first couple seasons. Most of the others hate him for his personality and behavior (which is part of the results of his abusive childhood) and he often acts in a way that droves away even those who starts to like him.