There is no predefined path to getting the good ending. You need to feed the Worm to make it grow and reach the next stage of its evolution, at 10 and then 15, and eventually bring it to 20, where it completes its growth and takes you to the ending screen. Each ending is more or less the same, but the text changes depending on Helen's mental state.
I am constantly updating the game to fix various issues, but here are a few simple tips:
Masturbate it first if it is in heat at the beginning of the day. If Asha says that it wants to mate, put masturbation in one of the other two slots, or both. Masturbation cancels the attack and can also make it grow a little.
Hide to recover mental health. If the Worm is hiding too, you are almost guaranteed to recover some mental health.
Observe: if you get one success, the Worm gets -1 to its next roll. If you get 2 successes, it...