I've just finished The Witcher 2 and I've noticed that there are some of elements that remain unknown to me, like does Geralt regain his full memory between the two games? The major events that took place during the 6-month period and how Yennefer also regained her memory, enabling her to send a letter to the witcher.

Geralt had regained his memory by the end of Witcher 2.
Yennefer regained hers earlier, with the "help" of the Nilfgaardian mages.

Hey there!

It's great to see you've finished The Witcher 2! That game indeed has a rich storyline that sets a complex stage for The Witcher 3. Regarding Geralt's memory, it's a gradual process. By the end of The Witcher 2, Geralt has regained much of his memory, but it's not entirely complete. The full return of his memories, including those involving Yennefer, becomes a key narrative point in The Witcher 3.
Hope this helps, and enjoy your journey through the Continent!