Thanks for the advice, I decided that I will go through with my promise of 3 dates, but I'm hoping she doesn't feel as great about it afterwards. Maybe I can talk to her friend and ask her to help me out?
Yes. Talk to her friend and have an honest conversation. I don’t know what kind of relationship you have with her or what kind of person she is, but is Rebecca is a decent human being worth marrying, I’m going to guess BFF is also. I’d listen to what she has to say first, then broach the subject that you aren’t liking this much. She might be willing to help you get your relationship on track. Remember, Rebecca chose you and she probably doesn’t want to lose you either.
Also, I’d ignore the idiots who say she’s gone or you’ll never have a sex life. I’m guessing none of them have been married. I have. The reality is no one knows how this will land and neither do you. She might enjoy the novelty value for a bit and then say “that was nice but I’m done now”. So, go for the three time experiment and see how it goes, then talk
Third, I absolutely would insist on being present. You want her to stay in contact with you, so I’d follow the advice of many cucks and keep it a shared experience at least for now. Only two more times.
Fourth, this is a bit of a wake up call for being better in bed. I’d train and learn some tricks. For endurance google the start stop method. I went from being a full on premature ejaculator to pretty near full control. I could last for hours if that was called for. It’s just about practice. Of course, I was also I. Good shape then. (Running marathons, etc) so start working out also. It’s like any other sport: you can train and practice.
Fifth, I might think about the power imbalance. I REALLY think the idea of asking for a threesome is a good idea. Maybe not the BFF, and maybe once you’ve trained a bit, but her watching you make some other woman scream might get her to see you in a different light.
Lastly, I would understand the difference between love and lust. For example, there are a lot of women I might enjoy being with and it might be exciting, but none of them are going to compare with my wife in all the ways that matter. So, understand that being sexually excited doesn’t really say much about her sense of connection to you.
In a relationship, it’s you connection that matters. Keep working on that. Take her on exciting dates (how about rock climbing?) and she’ll get a touch of excitement with you.
Good luck. I think this will work out ok for you, and perhaps even better than it was.