Let's see how some of the major battles ended:
In Season 1 Prophecy Girl, she got rescued by Xander then dispatched the bad guy but it took until When She Was Bad to get over the trauma. But she had faith in her duty, not her instincts.
In Season 2 Becoming Part II, she had to kill Angel because she could see that there was no happy ending - either the world would be plunged into hell or she had to kill Angel. She chose to kill Angel because there was no happily ever after whether she did or did not kill him and the rest of the world would have gone to hell if she did not, so she made the correct choice. Even if Xander correctly deduced that she would hesitate if she thought Willow could save Angel, so he told her Willow's words were, "Kick his ass!"
At the end of Season 3 in Graduation Day Part II, she has a plan ready to meet the threat of the Mayor. The plan is formulated, executed and it works - the Mayor is killed and the war on the vampires that were the mayor's army goes well for Buffy's side. She has confidence in logic and it pays off. She also communicates well with the student body, a capability we only see here, and gets them prepared for the fight. This season is the peak of her capabilities. But there was a problem: in Helpless, Giles put her through the cruciamentum, a test he knew was dangerous but as a Milgram experiment failure, he went along with it. I would not blame Buffy for shunning him after that or at least demoting him from father figure and trusted advisor to just another input to her decision-making process.
At the second-last episode of Season 4, Primeval, she faces and defeats Adam with the power of the first slayer transferred to her via the scoobies.
For Season 5, in The Gift, she sacrifices herself for Dawn after understanding that this would save her life. But up to that time she has risked the world for Dawn, someone she knows is not real.
In Season 6, Spike is right, she came back wrong. Nothing goes her way. The entity that comes back to life with her almost kills her. The house consumes money, she ends up at Doublemeat Palace, the Trio plays with her mercilessly, Spike and Buffy get into a cycle of mutual abuse with Buffy being the only person he can hit without pain from the implant and finally Dark Willow kicks her butt and she has to be saved by Giles and Xander.
In Season 7, the cumulative effects of Season 6 show their effect and she abandons logic altogether. As other threads have noted, in any rational world, her decisions would have resulted in immediate annihilation. Only bad screenwriting saves her. Giles is the one who sends Spike to the Guardian's home monastery to see about the scythe. Giles tries to diagnose Spike's trigger after Xander explains what a trigger is. Giles tries to dispatch Spike who is a loose cannon and only fortuitous writing saves Wood and disables the trigger.
I don't see any of this as trusting her instincts in Season 7. I see it as indulging her emotions, which is quite a different thing. There was an interview once with one of the Mercury astronauts where he was asked, "What would you do if you found a sudden problem that would kill you in ten seconds?"
The reply was, "I would use nine seconds to think and one second to act." Words to live by for everyone. But in Season 7, Buffy indulges her fantasies about Spike, disregarding the trigger (and having done nothing to find or fix it). Her instincts would have told her that Spike is a hazard and she would have to keep him chained up or in her company at all times and Spike agrees. Her feelings told her to give Spike a pass because he is the only person she can connect with emotionally. She obeyed her feelings, putting everyone at risk and giving everyone a reason for mutiny. Her willingness to go back into a battle where she is clearly outclassed shows that the tactical and strategic thinking she had mastered in Season 3 was now gone. This shows retrogression, not progression as a slayer. Her motivation is not faith or reason. It is pure self-indulgence.
