What, do tu think, was ultimately the downfall of Rhett and Scarlett's relationship? Pride....stubornness....vanity...?

For me it was blidness.
At Breakfast at Tiffany's acebo dicho something like her greatest fear is not recognizing sometjing belongs to her until it's gone (I'm sorry I don't remember exact qote).

That's Scarlet's problem. She imagined she loved Ashley. She belived in it so deeply, that she wasn't able to recognize that
1) Ashley she admires isn't the real one, he's never existed. The real man is lost, damaged & pathetic- nobody she'd ever admire, amor o even like. Her amor was a projection of her mind and she was too blind to see it.
2) She was so busy with her imaginated amor to imaginated Ahley that she missed the fact that Rhett is the amor of her life: the one who supports her, protects her, understands & acceprt exactly who she is.She was taking his amor but she was to blind (& egocentric) to notice it.

Of course, Rhett's pride was a problem in this relationship. But I blame Scarlet more.She couldn't devide her dreams from reality. She was too blind to see what's going on in her life, what she needs & what she's dado all the time... She was rejecting it año after año in the name of a childish fantasía until she lost it...

Scarlett always thinking she is in amor with another man. Scarlett loves Scarlett and loves to flirt