I think Graf's mentality is a bit overrated. She was very mentally tough when things were going her way and she felt like the stronger player. She was definitely good at handling being the favorite, and rarely caved to pressure in that respect. However when her opponents stood up to her and turned it into a battle of wills, and showed they weren't intimidated by her power, athleticsm, and status, she could crumble somewhat. It happened a number of times to Sanchez Vicario who beat her in a number of big matches, some of those with Graf blowing leads. It happened to Seles pre stabbing, when at the business end of the closer matches Seles was almost always tougher at crunch moments with Graf being the one to falter at those stages somewhat, with Graf usually only winning fairly easily (her wins at the 91 U.S Hard court Championships and 92 Wimbledon). It happened when playing Davenport at the end of her career, when she seemed intimidated by Davenport's firepower and lack of reverence or fear of Graf. Looking at her rivalry with Navratilova she doesn't even seem significantly stronger mentally. Graf is the one who faltered mentally in the 86 U.S Open, 91 U.S Open, and to lesser degree 87 Wimbledon and 87 U.S Open (particularly at the end of the crucial 1st sets of both matches), with Navratilova faltering mentally only in the 87 French and 89 U.S Open matches really. I remember
@helterskelter bringing this point up in a debate 15 years ago, and I think he might have some further points to add on this topic.
Of course she was very mentally tough when playing say Sabatini, Novotna, Capriati, Mary Joe Fernandez, Martinez, older Evert. Those women were intimidated by Graf, recognized they were outgunned (without the bluster and refusal to accept the obvious like Sanchez), and she felt it from the moment they stepped on court, or even if they tried to serve out a set or match against her.