Perhaps Sakura is dealing with family issues. She could be hiding the pain of having a difficult family situation, like an overbearing parent or a family member with an illness. In the fanfiction, she might focus on her school or ninja work to distract herself from her family problems.
One character could be Naruto. He might be Sakura's friend, but completely oblivious to her hidden pain. He's so focused on his own goals that he doesn't notice Sakura's struggle, which could add to her feelings of loneliness.
One possible reason for Sakura to hide her true strength in fanfiction could be for self - protection. Maybe she is in a world where showing too much power makes one a target. For example, if she is in a political intrigue - filled setting, she might keep her strength under wraps to avoid being used by various factions.
Her closest friend might help her. Someone who really understands her and is willing to keep her secret. For example, Ino could be that friend. Ino has always been there for Sakura in the original story, so she could be the one to support Sakura during this difficult time.
Maybe Sakura is afraid of the reaction from her friends or family. So she decides to keep her pregnancy a secret at first. She might be worried about how people will judge her, especially if she's in a difficult situation like not being in a stable relationship.
A common plot device is the 'big reveal'. Sakura's true strength is suddenly shown in a moment of great need, like when her friends are in extreme danger. This creates a dramatic effect and often turns the tide of the story.
I'm not sure specifically what that 'sakura end her pain fanfic' is about without reading it. It could potentially be about a character named Sakura who is going through some sort of difficult situation and finally finds a way to end her pain. Maybe she overcomes an illness, or escapes from a bad relationship or a tough life event.
One interesting plot could be Merlin hiding his pain from Arthur because he doesn't want to seem weak. He might endure the taunts of the knights in silence while secretly using his magic to heal his internal wounds. Another could be Merlin hiding his pain over his mother's absence or the fact that he has such a huge responsibility on his shoulders with his destiny to protect Arthur and Camelot.
In many fanfictions, the relationships can be very creative and non - canonical. Regarding Sakura being Pain's girlfriend in a fanfiction, it depends entirely on the specific plot and the writer's imagination. It could be a unique take on the characters where they are paired up in a relationship that is not part of the original Naruto storyline.