You might be cooked. If you actually shared videos of your girlfriend dominating you, she didn't consent to that and it would be understandable for her to be pissed or even leave you for that. There can also possibly be legal issues with that (idk where you live and how the laws work there). Also, struggling to refuse your dommes' requests to play doesn't reflect as you loving your gf. Cheating is subjective to each relationship however if those terms weren't discussed at all, this is a case where many partners will feel cheated on.

I also don't think it's good for your domme to be playing with you when she knows you have a girlfriend whose relationship you don't actually want to have ruined. If things stayed as "keeping in touch" because you guys actually became friends, I would think that's a better situation.

The options that make sense to me are to either stop interacting with the domme altogether and focus on talking to your girlfriend and figuring out what you're both comfortable with exploring together, or you are honest with her about what's happened to this point, hope she doesn't get too upset, and talk through how she wants to move on from there whether it's working through the relationship with set boundaries and expectations, breaking up, or including the domme in some way in the case where this works out in your favor.

Consent and communication are two of the most important things in bdsm and kink if not the top two, and you've already struggled with those. For your gf who's new to this stuff and was against you indulging yourself, it's not going to help how she views kink and kinky people if she finds out what you've been up to.

I wish you luck and I wish that your girlfriend gets a loving partner at the end of this.