I told my girlfriend I wasn't comfortable with her dating other people, but she did anyway

If my polyam girlfriend dates someone else at the same time as me when I've made it clear that I'm not comfortable with it, that it hurts me, isn't that cheating? She acts like I'm not even there, and it hurts so much.

If you have not given your consent, and you consider it cheating, then it's cheating. End of sentence. If your partner is doing something that hurts you and makes you uncomfortable, especially after you made it clear to her that you weren't okay with it, you should probably leave the relationship.

No one is obligated to change their behavior because someone asks them to. I could say to my partner "if you wear a green shirt, it would hurt me, make me uncomfortable, and I would consider you wearing a green shirt after I told you this to be a betrayal." Then, he could decide to wear a green shirt - he still has that right. But then I have enough information to know that we're probably not compatible, since I'm not comfortable dating someone who wears green shirts.

It sounds like polyamory is a dealbreaker for both of you - she needs to be in a relationship where she can date other people, and you need to be in a monogamous relationship. So the deal has effectively been broken. 

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