Why didn't Syaoran lose his memory?
Before the Sealed Card was able to attack and gather Syaoran's most important feeling, the Hope Card negated it. 

To explain it more clearly, it was because the nameless card nullified the power of the sealed card to obtain Syaoran's feelings. With regard to this, Sakura's powerful emotions towards Li made that happen. The Sealed card and the nameless card merged in the process, creating the Hope Card. That is why the Hope Card or the Love Card (which was called in the english dub) was the card who doesn't have a clow card counterpart, meaning it only exist as a Sakura Card because it was created by Sakura's feelings towards Li Syaoran.