I think it is so obvious that you should break up with her at the speed of light that the only question I have is the following: "Why is it not clear to you??"

Take a look at what you wrote:

"I don't even know why she wants to marry me"

Ex gf "knew how to keep me confident and the result was I was never ever insecure like I am now".

"I absolutely hate the person I am now."

You wrote all of that - you did. And in the post, you were only one step away from explaining the root of the problem. First, you described the lack of staying power. Second, you noted that with 2 prior partners, you did not have that problem. Then you wrote about the "only tip there". Then you wrote that ex gf kept you confident and you were never insecure. Then you wrote that now you are always insecure. And you wrote that ex gf genuinely wanted sex with you.

From that it follows as the most likely hypothesis that the put-downs by the fiance and her failure to initiate caused you to lose confidence and become insecure, and that, in turn, caused you to develop a performance anxiety which results in the lack of staying power.

Why did you not connect the dots? Are you severely depressed? Because with the settling down and finances what her ulterior motives seem to be, you were spot on. But you did not conclude that you should break off the engagement.

Since you have only known each other for 6 months, the engagement probably has not been widely announced, but if it has, here is a complication of etiquette tips for your very situation. Good luck! Btw the way you describe the ex gf makes me wonder if she perhaps is available again? She sounds like a someone with a good heart. Unlike the current fiance.