Just the general situation for Syaoran. Every time Sakura regains a memory of her past, but he isn't in any of them, even though he remembers everything that happened perfectly, you can just see his heart getting ripped out. Added onto that, Sakura's desperate desire to remember that "someone" who she can't possibly recall no matter what, even though she knows that person was so important to her. Even before Cerebus Syndrome set in, this series was heaps of heart-breaking.
During Acid Tokyo when the true nature of C!Syaoran is revealed and Sakura is begging him not to go after she receives the feather was too much.
"There will probably be more times when I will have people hurt themselves or steal something for my own selfish reasons. And I'll probably be punished for it. As I did for doing what I just did... But... Even so... I want it back for you! I want your lost soul back! Syaoran-kun..."
And let's not forget when she comes back, so hurt that she collapses, and yet the first thing she does it to apologize to Fai because she couldn't help him when he was in pain. Fai wasn't the only one who cracked there.
"I'm sorry. You were hurting so badly... and I couldn't help you at all! I'm sorry! Even now... I'm sure you... are hurting far worse than... than me... Even if that's true... I'm just so glad... that you're alive... Forgive... me..."
That is exactly why the opening of Tokyo Revelations is so ominous and foreshadowing. You just get this feeling that it would not end well.
Three words: "Good morning, Kurogane."
The moment during the showdown with Ashura, when Fai screams that he just can't bring himself to kill the first person ever to show him kindness.
Fai's face when he thinks Ashura just killed Kurogane.
Chapter 210, in which C!Syaoran dies. It isn't until he says "Kurogane-san... Fai-san... Mokona..." that it hits home that this isn't the unstoppable, murderous clone that's dying, but the Syaoran we started the series with.
Also, in the same scene, as Syaoran is dying: he returns his stolen magic to Fai, who says "It would have been much much better if you had just come home." That's right, this is the boy who betrayed him and tore out his eyewith his teeth, and Fai is still willing to forgive him and just wanted them to be a family again.
The twins Fai and Yui have been condemned to be imprisoned for eternity without even each other for comfort. However, they are given an escape clause: one of them can die and free his twin of his misery. In response, Yui and Fai look into each others eyes, and without saying a word they clasp hands. It's the last moment they ever have with each other, until they lock eyes the moment before.
When Sakura wakes up for the first time and see Syaoran, for her to say "Who are you?" to Syaoran due to the price is just heartbreaking if you remember that before, they were very close friends with the princess about to confess her love to him only to be ruined by the guy who set the entire Tsubasa plot up.
And when it finally cuts away back to Syaoran, (after the book is wretched out of his hands) the first thing he does is apologize to Kurogane—in tears.
Also the callback to Kurogane's backstory in Celes. Tomoyo closes his mother's eyes and tells him to let her rest, and that's basically exactly the same thing Kurogane tells Fai to do for his twin.
Remember this? At the time it was a really cute thing for Fai to say, until you later see his backstory and how he learned to Stepford Smile like a champion from Ashura. Then it wasn't so cute anymore.
"There is something you can do. You can smile. Sakura-chan, your smile is like food to a starving man for Syaoran-kun."
"...like food to a starving man..." showcases the sheer desperation and pain of Syaoran because Fai, of all people, knows very well what it feels like to starve (remember the state he was in when FWR talked to them?), so the comparison is not made lightly.
In Nihon when Clone Sakura is stabbed, by Clone Syaoran of all people, and dies in his arms. She wasn't even able to say 'I love you' before she died. And Clone Syaoran's cries just make you want to cry.
Episode 08 we see Chu'nyan desperately running after her mother's lifeless body as Tambal banishes her to another dimension. Then she gets a brief reuniting followed by a departure with her departed mother.