Stormen said:
Don't worry, Guts won't leave Casca as she will leave him first. Before being overhyped, do keep in mind what the skull knight said "What You Wish For, May Not Be What She Wishes For"
Be ready for some kind of disappointment, or potential heartbreak.
Leave him? After all he and the others went through to get her there and keep her safe along the way? She is not that selfish. Based on what we've seen in the past few chapters we know that Casca understand Guts is suffering and that he is protecting her.
I've interpreted SK's words as "she might not want to be sane again because she will suffer more that way." But it seems Casca is ready though, she already said she wanted to see a certain person. I don't think they will go their separate ways anyway. They are bond together, and if Cascas love has died down it will probably spark again at some point.