Call Of Shadows
Sanity loss isn’t permanent. There are many types of treatment in the modern world, even in the 30’s, which are useful for dealing with a damaged mind. The most common way is to see a therapist and have them counsel you and perhaps provide you with medication. For the purpose of the home game, I will also allow you to regain lost sanity by producing works using the Craft ____ Skill. For example, if your character enjoys sculpting, painting, writing fiction, producing music, etc. They can work on producing new material as a way to regain sanity lost from their experiences.
The maximum sanity reduction that comes from learning about Cthulhu Mythos cannot be regained in any way.