Fai didn't feel he had opened up too much because Kurogane wouldn't let him die... He says explicitly that he felt he and Kurogane had gotten way too close and if he forgave Kurogane they would get even closer. Fai has a lot of trouble with emotional attatchments, he's always wanted to be this aloof, closed off person so that no one ever reached in too closely and saw who he really was. Sakura, while they are close, never prods Fai about his past or what he is hiding. The only person who has bothered to do so is Kurogane. He does feel that to a certain extent Kurogane has betrayed him in going against his wish, but I don't think that is the main reason he is pushing Kurogane away. He doesn't want to get close to anyone because he fears that they will be hurt if he does, therefore he will push Kurogane away as much as he can because he feels that if he doesn't, their closeness will develop further and then people will get hurt. His closeness to Sakura is different from his closeness to Kurogane. Perhaps he feels that there are so many less strings attached to being close to Sakura in a parental way than it is getting close to Kurogane because he is an adult (and deep emotional attachments between adults involve a LOT more). I just don't know how more explicit the lines:
I didn't even realize I had crossed the line I'd drawn.
...So, I can't forgive him for choosing to make me live.
If I do... I'll be getting close again.
can get in terms of what Fai is intending to do by pushing Kurogane away.
About that... Fuuma was an insane psychopath trying to destroy the world in X, but he isn't in Tsubasa. What CLAMP is doing is giving characters the SAME SOUL, but not necessarily the same exact person throughout dimensions. If that were so, Sakura would be a Cardcaptor/Master of Clow Cards in every dimension, Tomoyo wouldn't be able to do magic in Nihon, Kendappa, Souma, Ashura-ou, Ryuu-ou would be dead and Yasha wouldn't, Kamui and Subaru wouldn't be vampires OR related, Seishirou and Fuuma wouldn't be related either. So therefore, it is very easy to see that just because Shougo was a bad guy in the X movie, he doesn't have to be one in TRC. Same way that Primera doesn't have to be in (one-sided) love with anyone, and Chii doesn't have to be a computer. They're all inherently the same, but they are still a lot pf differences throughout dimensions. It seems the only thing CLAMP isn't changing are soul mates and basic personality (Kamui's bratty, Subaru is angsting, Tomoyo still fangirls Sakura and is kind and gentle, Sakura is still hardworking and loving, Syaoran is still determined, Touya still teases Sakura and Yukito is still kind... etc etc).