It's pretty dogmatic to assume that nobody who CTBs ever might have regretted the decision. History is full of people who obviously regretted the decision with evidence like calling 911, bemoaning the decision to bystanders, or otherwise trying to save themselves. Sometimes it's SI, sometimes it's the realization that their attempt was inspired by loose reasoning and they really do want to live. On a personal note, yes, I did have a couple friends who regretted halfway but they did survive and were teenagers who admitted the attempts were a cry for help.
Of course, most people on this site crave death and DON'T have options to CTB, so you tend to encounter the assumption that everyone who CTBs 100% meant it and needed it... because we've spent so long fighting against the anti-choice sentiment that suicide is a permanent solution to a temporary problem or that 100% of attempt survivors are soooo happy they're alive. Just saying.
So imo sometimes yes, sometimes no. Certainly there are many those who successfully CTB on the forum who are very happy with the choice. Personally I want to live HAPPILY but choose not to live in the MISERY I know is inevitable thanks to life's shittery. Thus I'd more regret that things had to turn out this way but not regretting I was about to die.