YouAreHere is wise; give heed to her words.
I wonder, hellokitty, if your post might not have been a vent to some extent. If venting is all it was, then you probably don't need to change your situation much; you can just tap into Polyamory.com sometimes for a listening ear when you need it.
The one thing that might concern me is, how much time and attention are you investing into communicating with your girlfriend? not just about little things, and not just about pleasant things, but about things that are heavy and serious and hard for the listener to hear (even if the speaker speaks gently). In other words, are you telling her that you feel stifled, confined, muffled, or something on that order? Do you desire a more free/outgoing sex life with other people, and you're holding back to protect her feelings?
She might never feel like she can stand to let you have the amount of freedom that you want, but I still think it's important for her to know that just as she struggles with the idea of you having a "boyfriend on the side," so also do you struggle with having "only just a boyfriend on the side." No use pretending everything's hunky dory when you're feeling bad for whatever reason. If nothing else, she needs the opportunity to express due sympathy toward you.
Now that doesn't mean you should blast away at her and say, "This is all your fault! You're obviously a jailer at heart." There's no bad guys here, and there's no blame that should be assigned to anybody (to you, to her, or to your boyfriend). As we've seen from this poll and thread, some people are suited for a very free style of sharing their love, while others are better suited for lots of limits and controls in their love life. It's not bad to be either kind of person. It's just who you are.
So when you talk to her, do so kindly, diplomatically, and compassionately. Put yourself in her shoes (while giving her a chance to put herself in yours). Listen carefully to whatever she wants to say, even if something she says stings you. Don't use "her talking time" as an opportunity to think of a punitive riposte. Instead, use it as an opportunity to try to see into her mind and heart. Repeat back to her whatever she just said, in your own words, so that she knows you're hearing her, and express empathy so that she knows you care. Then you'll probably have a good opportunity to share your own thoughts and feelings. She'll be more "open" to hearing them.
Some guy -- was his name Gary Smalley? -- did a presentation on marriage and relationships some years back. One interesting concept he presented is that, when a person feels hurt, their soul often clenches up like an angry fist. They're protecting their vulnerable insides with an angry exterior. So when you see someone acting angry, it's very possible that a patiently, persistently gentle way of approaching them will slowly convince them to relax and unclench that fist, and trust you with the trembling wound on their soul's defenceless palm.
Keep that in mind whenever you try to communicate with a loved one -- or even just about anyone. Anger is a shield people throw up when they feel hurt and wounded. When someone lashes out at you, try to defer your own hurt and instead just back away and then re-approach them with a tender touch. If you persist, you may be able to convince them to relax and unclench that fist so you can do first aid on the bleeding part of their palm. At that point, what might have turned into a nasty confrontation, instead evolves into a moment of healing and bonding.
Anyway, approach difficult communication with that technique in mind, but don't avoid approaching it at all. The tough conversations have to be had.
Who knows whether you'll eventually have to split up with this girlfriend? That might be more healthy for both of you. But don't give up quickly; give time and communication a chance to at least reveal whether the two of you can be compatible enough to stick together. Or if you must part, try to do so as friends.
I'm sure your story has (more) details that I'm (goofing up or just) not taking into account. But that was one thing I thought I spied: the possibility that a bit more (and better) communication could help both of you to make peace with the differences in your personalities.
Oh and have faith that once the other person's fist relaxes into an exposed hand, and you've helped nurse their wound, then it's quite likely that they'll then reciprocate and help heal you too, as much as they're able. So even from a "selfish" standpoint, there's a direct reward in it for you too.
Not that such techniques always have a happy ending. For example, some people have been hurt and angry for so long that their hand is permanently clenched into a fist. There are some people who can't be reached. But give your girlfriend a chance to hear your hurt, share her hurt with you, and allow the both of you to be patients and healers to each other.
And I know that only covers one small facet of your dilemma, at best. But I'm hopeful that some small part of this post will prove useful to you.