Thanks for the timeline. Nicknames for your partners would help us to give feedback. Paragraph breaks are nice too.
I'm not quite sure I understand the dynamic here. Let's break it down.
Years ago, my partner of now 15 years, decided we would like to try new things in the bedroom and started to explore swinging (always together, always with other females, no males) and respected these boundaries and never crossed a line that we both decided on.
So far, so good.
Four years ago we met a girl and started fooling around with her about 2 years later. I was happy to leave it at friends with benefits.
So all three of you were having sex together. Both women are bi? It was fun, light, casual. And the threesome sex went on for a while. It was different than at the swinger's club though, intensity built. Feelings arose...
...but then they started hounding me about calling her our girlfriend; this was not something I was comfortable with, and still am not. Somehow in there they both decided that we were all in a relationship. I was not part of that decision. I have made my feelings clear.
So somehow, against your will, the other couple (your bf and your female FWB) wanted to make it "official." They wanted to call it a "relationship." And they forced(?) you to agree that you and the other woman were girlfriends in a romantic sense? But you don't "like her that way"? You're sexually attracted to her. But maybe you feel she's a threat to your continuing relationship with BF? You like her as a friend, but you don't want romance. You maybe fear she's trying to get your bf away from you.(?)
About 7 months ago they slept together and hid it from me.
But all three of you had been having sex? Threesomes only with you and him and her? But you and bf could have one on one sex? Gf couldn't ever have one on one sex with either of you? Was that the original agreement? But then bf and other woman decided they were going to have one on one sex whether you liked it or not?
I don't feel good about this, but about 2 months ago I went through his phone and found two videos of them having sex, once was the time i was not aware of, the second was one he "swore he told me about." He refused to have her stay away, for me to heal, and [she] has been at my house every night since...
Is it your house or your bf and your's shared home? You get to say who is and is not allowed to be in your home. If you don't like her being there every night, say so, and enforce it.
I go to bed because I work early and they stay up late together. I have had a couple instances where I can't sleep, and go downstairs, and there is something going on - one time he said he was "going to let her touch it" and I went downstairs to find a full bj going on. Last night her tits "accidentally" fell out of the shirt I had bought her earlier.
You're buying her shirts? That implies you like her a lot, to be giving her gifts. But it seems you don't like them having one on one sex. Is that the main problem? Do you want to keep having the threeways, or are you mad at both of them and don't want to have sex with either one now?
If bf and his gf are attracted to each other, they are going to have sex. That's what adults do. You really don't have a right to tell any other adult who they can or can not have sex with. But you are becoming incompatible with both of them. You don't want a full on polyamorous triad. You also maybe feel neglected by bf? Are you sexual needs going unmet? Are you angry because he doesn't seem to care that your desires for this relationship are different than his desires or goals, but he's just going along doing whatever he wants? Is it time to call it quits? If it's your house, you can kick him out so he can go bang and love on the other woman. If both of you are on the mortgage or rental agreement, it's trickier.
I don't trust or like the situation, but I don't want to hurt my partner by casting her away, but I'm tired of being suspicious and jealous, and in general I'm tired of hurting. Any advice?
Make your boundaries clear. Decide what you want. Talk together like adults. You can "cast her out" of your life, but you can't literally "cast her out" of his life. You can "cast her out" of your HOME though. No one needs to allow anyone they don't trust into their sanctuary.
Tell them you're tired of these "games" where he might let her touch it, her boobs somehow fall out of her shirt, and somehow they are intercoursing away in YOUR living room after you go to bed! Tell them that is unacceptable. If they must continue, you decide what YOU do next, as advised above.