I'm not sure what kind of communication you two have. Was this just a "I need to do this and you better be ok with it or we're not a thing. Oh? You are ok with it? then I'm gonna do it and that's the end of discussion."
I don't have enough information to give you a "unique" reply to your "unique" situation, but I will be my usual glib, judgmental self and tell you what I think in the absence of the other person's side of the story.
I do not think you should have to just suck it up (i mean, "be introspective") and deal with it. I have noticed that often, the partner who is pushing for a poly/open relationship and/or the partner who finds another relationship first often behaves in a way that is very dismissive, insensitive, and inattentive to their "established" partner. People on this forum dispense advice that is often supportive of the person going through NRE and counsel the person left out to work on themselves and get to the root of their problem. What is not as often acknowledged is that the "NRE person" (the one with the new partner) DOES often behave in a way that takes their existing partner for granted.
Is your girlfriend still showing you by words and actions that you are important and that she still loves you? Or does she see this as something she is "going through" and that YOU need to support HER? I believe that it is on the person who wanted to be poly/finds someone else first (this does not apply to those who were non-monogamous from the outset) to treat their original partner extra-special and go out of their way to check in and see if there's anything they can do to make it easier on the original partner. This isn't about primary/secondary hierarchy or "couple's privilege". It is about being sensitive to the feelings and needs of someone you care for, and realizing that this isn't all about you and your NRE and how HAAAARD it is to juggle two relationships.
If your girlfriend is not at least TRYING to show you affection and understanding and maintain your comfort zone to a certain degree (this does not mean keeping score of how many times she gives you sex versus how many times she gives him sex, or anything like that), then she really needs a reality check and a wake-up call, and needs to SHOW you she still loves you, or she has no business seeing this other guy and still expecting to be in a relationship with you.