Full stop, this is what you need to resolve, yesterday...because if I'm being completely honest, what I'm hearing is, "I prefer a monogamous relationship with my partner, but I don't want to accept the consequences or the societal stigma of not allowing openness in my relationship."
You can't have both.
"Open to new experiences" is generic bait offered by not so adventurous folks whom don't want their personal boundaries preselected out by attractive adventurous folk. It's not malevolent, but it's deceptive, manipulative as well. There are plenty of ways to find out the intricacies of threesomes and open relationships that don't involve actually engaging in them, but I think deep down you knew how attractive your probable participation in one would be against simply wanting to know the mechanics of how they worked and you made a conscious choice (even with the best of intentions) to sweeten your pot, as it were.
I personally understand the, "aren't I enough" question, but the moment you vaguely defined your "enough" as being game for this, I think you forfeited the privilege of asking your partner that...at this point I think you need to meditate and come to your partner with a definitive stance as to your willingness to participate in a threesome 1st, right now as is, and ask them if nothing were to change from that could they stay, without trying to change your mind. Might be the end of your relationship, might be an opportunity for your partner to demonstrate a level of compromise and cooperation that will make you love them even more....but it has to start with a definitive yes or no from you without any case building at play if you want it to be true to yourself.