Severus had one thing well he has many things too, hide, but one thing he never told anyone not even Lily his first friend. That he has an illness.
His worst fear was for her to treat him as if he was not able to take care of himself. His mother looks at him with deep sadness and fear of not knowing what she can do to help him. His father is ashamed of him and can barely look at him without disgust. His parents will fight about what they should do with Severus.
The day he meant his first friend who turns out to be also magical, he felt connected to another person for the first time. He felt like this person cares about him, Severus did not feel alone anymore. He finally had someone in his life. However, the fear of his illness kept him from being honest with her. He saw how his parents treated him, he did not want Lily to leave him or treat him differently.
When the Day comes close to both of them will be going to school together, he felt great. Until he realizes they will be separated which she had to calm him down by promising, it did not matter the houses they will end up in they will always be friends No matter what.
However, as time went on, as she grows in popularity, and he who grows to be the victim of Bullying based on his looks and poor state his family is in. He starts to lose his trust in his friendship, a friendship that he held dear to his heart.
Until he realizes the very person, he called his best friend never truly defend him, she even took part laughing when she thinks he was not looking. And she even at times defended the people who do the bullying.
He did not know how to handle it, however, maybe if he had told her, she would have been there a little more. Or she would have left him from the very beginning.
However, maybe it was always a one-sided friendship from the start.
It is now best to move on and grow. It time to say goodbye to Lily. It time for him to seek someone who can help him with his illness instead of hiding it.