While i don't believe that the gang we're right in their handling of Buffy, i do believe they had a right to say what they said.
The problem with this group is, as close as they are, they constantly keep secrets from each other, and this is where the lies. This is how Spike managed so effectively to break the crew up, in the Yoko Factor.
Another thing people forget, even though these people fight demons and act all adult, Willow, Xander, Buffy and Cordelia are still teenagers. Still struggling with emotional,personal and familial issues, and they do believe that they are alone in the world, as most Teenagers do, hence the need to resolve their personal issues.
This also helps to understand why Buffy took Joyce's threat to heart, simply because she has no real frameof reference, other then to take what her mother says as fact.
As for the whole 'Buffy not holding onto issues'...well Like Epo pointed out, there was Faith...granted thats an extreme case, but Buffy, could have tried a little harder to understand where she was coming from....and in Dirty Girls, theres still some animosity towards the 'Rogue Slayer' even though she has served time for her crime AND returned to the fold of the Slayer.
Another instance showing Buffy, may forgive(to an extent) but NEVER forgets, and does use the 'throwing in your face' tactic in Selfless.
XANDER
No, of course not. You think we haven't seen all this before? The part where you just cut us all out. Just step away from everything human and act like you're the law. If you knew what I felt"”
BUFFY
I killed Angel! Do you even remember that? I would have given up everything I had to be with"” I loved him more than I will ever love anything in this life. And I put a sword through his heart because I had to.
WILLOW
And that all worked out OK.
BUFFY
Do you remember cheering me on? Both of you. Do you remember giving me Willow's message: Kick his ass.
WILLOW
I never said that"”
XANDER
This is different"”
Now, that incident was over 4 years ago, and she still remembers that single line, even though she had so much going on.
This indicates to me, that played on her mind time and again over the years, only difference, she bit her tongue, until she was in a position to use it.
Now, i'm not saying she was wrong to use it, but, to say she does forgive, well, i don't believe so, not entirely....and thats what makes her Human.....
Another time she is pissed off at Giles, when she is turned back to herself after the Faith-Swop, she is annoyed that Giles didnt recognise that 'Buffy' wasn't her but Faith in disguise, even though a few weeks before, Giles had turned into a Demon and she recognised him......