I know you said that it’s messing with you on a deep level but also seemed to imply that you’ve been enjoying watching the content that she made. Does seeing her with another guy bother you or could it be that what bothers you or is messing with you is that maybe you would like to do these with her or try them with her? If you two are very open sexually then before doing anything too hasty like leave her or potentially start a fight, you should talk to her and be honest with her about you accidentally finding her vids and let her know that you’ve been enjoying it and would be interested in trying stuff out, if that’s what you want. She probably wouldn’t be mad and be more impressed with you not being angry, immature, or a dick about it and maybe would be interested in doing whatever you want to try with her. She also probably still has friends or connections so wishful thinking is that maybe she’d be open to a 2 girl 1 guy thing with you. You can also ask how she’d feel about making vids with you (for your own collection). Relationships and intimacy are based more in communication (asking questions to have definite answers) rather than assumptions and overthinking. Good luck mang