Poly affects everyone differently, and it's common to have a harder time than you think you will at first, and for logical ideas about how things will work to melt away in the face of a complex and emotionally impacting reality. Your experience is a very familiar one here. It can and almost certainly will get better. Being honest about what you're going through is one of the best ways to make sure that it does, combined with doing a lot of hard work, which it's clear you've already begun.
Your husband needs to realize all of this, and to understand that you are you giving all three of you -- him, yourself, and the girlfriend -- a huge and difficult gift by actively processing what you're going through and asking for what you need. After all, just imagine the blow-up and the fallout if you'd just sat on all of this until you couldn't take it any more and felt like the only solution was for one of the relationships to end. All too often that's just what people do, sadly.
If you need to break down a little that's ok, and if it makes him uncomfortable to see that, well, too bad! As long as you're truly working on your feelings it's his job to toughen up and be there for you through it. You crying does *not* mean his relationship with her has to end, hopefully he can get over his guilt about that.
He also needs to understand that this -- "he assumed that all we talked about in theory would fall into place once put into practice" -- is never, ever a good idea in poly. Communicate, communicate, communicate.
Personally, for the record, even though my girlfriend is completely free to take another lover or partner if she wants, I'd be freaked the fuck out if she suddenly started referring to some random dude as her boyfriend, so I think your feelings are completely reasonable.
It might make sense for him to be talking to you at every new juncture. Like, before any new step in the relationship with the girlfriend he could be checking in with you first. That may seem like it'd end up with him feeling micromanaged and you feeling overwhelmed, but it could be worth a try for at least a little while as you learn what is easy for you versus what is hard... and it'll give you a chance to process before you're confronted with something new. Now what I'm describing is possibly easier said then done, but I think it's worth considering as an alternative to how things have gone thus far.
Btw, if you're not comfortable with him sharing explicit videos with women he doesnt know well it doesn't matter that it's already happened, you can ask him not to do it again. That is a perfectly reasonable request and another example of something that he *really* should have talked to you about first.
I guess it seems like I am being pretty hard on him? I'm sure he means well and is a great guy. This is just a matter of him learning that poly does not mean you get to do whatever you want... quite the opposite, especially at the beginning actually -- "go at the pace of the one who is struggling the most" as RedPepper and others here like to say.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.