lol I didn't mean to take over this thread, sorry! I'm no prophet, it's just an interesting situation.
You're free to do what you want, even if it means breaking off the engagement. So is she. If something terrible is going to happen, obviously it's better now than in a divorce court. But you say you expressly told her to give this a few goes before the two of you make up your minds. She's enjoying it aparently. You're tore up right now but at least partly enjoying it--right? And you may learn to enjoy it more.
So my best guess for you (and her) is to stay the course with your original plan. If that meant three or four hookups, or three or four different partners, whatever, go through with it. She's expecting that much freedom now, and you pushed her to agree to that in the first place.
After this initial experimental phase is over, you will have fulfilled your obligation, so to speak. She'll have got the sexual experiences you promised her and she agreed to. THEN, have a totally sincere heart-to-heart, and tell her everything that's still bothering you BY THAT POINT. So maybe by then you've learned to trust her loyalty better, you've learned to relax and enjoy this and let her have her fun. And she'll know better how much it excites her, or if she's just sowing wild oats and ready to take a rest from it. You both will know better then what you need from this, or if you're both capable of putting it aside (if either of you wants to do that by then).
At worst, there will be less judgment or guilt between the two of you if you have a fight about it AFTER you go through with the original agreement. If you pull the plug right away, you're taking away from her the freedom you urged her to accept in the first place, you'd be changing the conditions you demanded from her at the beginning. And you haven't let your own feelings cool down and sort out yet.