The Gemara says it as fact about Dovid Hamelech, not just that he thought so.
I happen to be coming from that Maharal that apashutayid mentioned. You asked where the Gemara mentions that Reb Meir said Libi Chalal Bekirbi. The Gemara does not say that he said so. I explained that he reached the level of what the Chovos Halevavos talks about, which is found in the Gemara with regards to Dovid Hamelech.
You and EiseneKop are saying that she failed faster or easier than him. That is not the point. In fact, none of them failed. A point was proven to both.
Besides, you should realize that doing an Aveira because of your desire is one thing. Many men do that, IIRC. The point of Da’atan Kalos is that she had no desire on her own, she was won over.
Furthermore, Reb Meir did not succum to anything. He was merely shown the Yetzer’s power. However, the rule that Torah protects you from sin, that worked for him until then, continued to work for him. The Maharal explains that that is what is meant by what is says that he climbed half the tree, and Rebbi Akiva went across half the river. They did not actually get close to the sin, but they felt its pull. The Torah did, indeed, separate them from the actual sin.
So, no one failed. He was shown the actual power of the Yetzer, and she was shown the power of convincing that can have an effect on women. It was not a failure on her part, it was a physical shortcoming.