I just want to add that if you do marry her, and, as you are correct to predict, she starts sleeping around, you should not attribute that fact to the size of your penis. This girl clearly assigned you the role of a stability pillar in her life, and nothing else. So in that sense, given that a pillar has a phallic shape, you are the strongest of all her male partners
. But she has not assigned the role of a lover to you so that role would then need to be filled by another, or many others.
You do not have to participate in her plan - you can break off the engagement - but if you willingly submit yourself as a participant in her plan and design, then at least "get" what is going on and refrain from making incorrect conclusions.
It is possible to figure out what she wants and what kind of person she is in more concrete terms, but I am not sure if you'd be able to pull it off. I think a complete disengagement from her and just licking your wounds of unrequited love in solitude would be far safer options for her. But if you want to try figuring her out better, you need to stop being precisely whom she assigned you to be - the main pillar of her stability. What is the opposite of stability? Uncertainty.
So if you postpone the wedding, that would be awesome. why so? because following through with the wedding AND calling it off entirely both provide concrete answers, i.e. stability. postponing the wedding introduces uncertainty and equivocates.
Stop pursuing her sexually. Stop being needy. Stop all of that.
Move out if you can.
Get a timer application if you do not already have it.
If you receive any kind of a communication from her (SMS, email, VM) set a timer for 2 hours. When the timer rings, answer the communication.
Next day, set the timer for 2 hrs 47 mins. The day after, for 1 hr 51 mins.
You get the point.
If she asks you for something that requires a time commitment at a set time, never commit right away. Instead, say that you need to check your schedule/
I do not have time to write much more and am positive that you got the gist of it.
If you can pull it off, then you might learn a lot about this woman.