This is the transition phase. It's hard, but you're going to be okay. She's not going to be great at managing her self (and her texting) around you until she's also learned her way through the transition phase, but you can tell her that you're feeling overwhelmed by the speed of everything. She may not slow down, but at least she can then be aware of your feelings during this period of adjustment and you can actively talk about how she can still show you love and respect while also being excited for her new beau.
You're now discovering it was never going to be just like the script you had created in your head, and you'll need to accept that. You aren't a movie director and she and her beau aren't your actors to direct. You'll need to let go of your expectations, and learn to roll with it.
You seem a little caught up in what they may or may not be doing physically/sexually. Again, this should pass and you will come to appreciate their autonomy and privacy, just as you will want yours when you start dating someone new.
It can be a big leap from monogamy to open relationships, and perhaps on to polyamory. Make sure you are doing things that foster your autonomy, too. Have a read of this and see if you perhaps rushed through this bit:
https://polyamory.com/threads/most-skipped-step-full-article.157088/
Have you got someone, a friend, who you have told about this transition? Someone you can call when your feelings are intense. Someone who will help you with the anger in the moment, because I'm sure you don't want to direct that poorly. Maybe you could put 1300 78 99 78 in your phone for those times.
This is still really new, and the new normal will be a while off, but you've made this agreement to open the marriage, for whatever reasons you had between you and her, and once you both adjust to managing the transition, it will become just...normal...and not something to get angry about or jealous over.
Speaking of jealousy - this is a fear of loss, where as envy is a fear of missing out. Have you and her taken the time to talk about what you're losing by letting go of monogamy? Have you talked about what you're fearing missing out on when the other person begins dating? Have you done much talking at all about the feelings, the logistics, and how you can be courteous to each other (such as when it can be phone down times so there isn't so much texting happening in front of you, and vice versa when you start dating and she has to go through feeling what you're feeling now?)
https://www.goodreads.com/en/book/show/17627888 has some good conversations starters in it.
Perhaps you could talk with her about this
https://www.kathylabriola.com/articles/are-you-in-poly-hell
I hope you're both communicating about this transition, but also having times where you
don't talk about it and enjoy each other's company without it always returning to the transition from closed to open. Keep finding the joy in your own relationship, after all, surely you didn't open your relationship as some kind of "fix" for it! (If you did, then why bother? Just go get that divorce lawyer, serial monogamy would be so much easier than an open relationship). So, given that your relationship surely isn't fubar'd, then don't let it get that way. Date each other, too. Text each other about more than household maintenance/bills/kids/extended family holiday planning/etc. Text her about pina coladas and rain...i.e. don't let the romance between you die because you opened up to non-monogamy.
She'll be right, you're just going to have to do the inner work to let go of the mono-normative narrative and the bullshit that jealousy=love. Most of us have had to; we were taught it from a young age, but don't let any romantic notion of our parents or grandparents generations being the perfect monogamous husband and wife model trick you either. Non-monogamy is pretty common, you're just embracing a model of it that aims to remove infidelity. Kudos.
