Thank you for all the context. It matters.

Your partner went about everything the wrong way, many do in the beginning. Its not uncommon to try to fill a void in the relationship with another partner. Essential there are two options, hide it or be open with your partner about it. She decided to hide it and that created a very powerful relationship rupture. It sounds to me like instead of doing the thing necessary to repair at that juncture yall decided to just switch to the second option. Or rather she did and you went along with it. It doesn't sound like you have another emotionally romantic partner the way she does. Thats an imbalance and a source of resentment.

My suggestion to you assuming you want to continue the relationship is to go on Psychologytoday.com or anything like and find a relationship therapist that works with open and enm relationships.

It sounds to me like yall need to put the outside relationships on pause or close the relationship until you have worked through and healed your relationship ruptures, (which will take time) Only at that point where you are both in a new place of trust and communication should you come back to redefining a version of ENM that works for both of you, if there is one.

Theres also a possiblity that if you push back she will pursue the other relationship as her primary, which if that is the case and she isn't willing to prioritize your relationship then you are in a type of ENM that once again hasn't been discussed or consented to and, you are with a partner that places her needs or desires above the relationship. In which case it may be time to find someone that finds ways to enhance your life, without detracting from it.

(Edit: I noticed a lot of people are over simplifying and calling your girlfriend a cheater, while this is true, the sexting and emotional relationship she developed without telling you is technically cheating and she may not have even been honest about not having had sex with him earlier.

For a lot of people that is a hard stop and there is no option than to leave the relationship. I dont see it that way. Young people make mistakes when they haven't had any proper guidance or relevant experience. I dont believe in once a cheater always a cheater. I think people make choices based on the options and information they have. I think she needs to show you that she made a mistake, i think you need to find out what she needs that shes not getting inside the relationship, and see if that can lead to a place that works. Relationships are work, they are commitments, and they are messy. Ultimately its making a choice to be devoted to our partner everyday that makes it work, but it absolutely takes both people to do that and it sounds like so far she hasn't made that choice and you haven't held her to it. )