How well do open relationships work?
From my experience, they rely solely upon 2 things: The maturity of BOTH (or all) involved, and the fact that BOTH partners seek the MUTUAL desire which is to be SHARED.
"Sex" is just that...sex. A physical means of obtaining and releasing sensual pleasure. It's a wonderful thing....So long as those involved share the same. Once it goes beyond the physical act itself, and the emotions become involved, it no longer is "just" sex, it becomes much deeper and intimate. It then is binded by emotion, which then becomes removed from "open" to something which can only be shared in a closed union of love, (which, in my opinion, is then the even more beautiful act of making love).

From the sound of your situation, there is no way in God's name that "open" anything is even remotely close to what you have with your present girlfriend. Because of that fact you can kiss obtaining that fantasy a grand farewell as it will not happen....not with your present girlfriend and certainly not with your (*assuming lack of) maturity.
(*You are clearly on "your own" lust hunt).

One cannot willingly enter a mutually monogamous relationship with intentions of it being as such, but then later down the line, (once confronted with a sexual invitation to your liking), change your mind about how you decide to behave without some serious, painful consequences.

An "open" relationship BEGINS as such: OPEN...from the very beginning. BOTH partners seeking the same mutual desire. There's no surprises later down the path that one has hidden motives, or whatever. That's irresponsible and even more so selfish of whomever hides the truth, (as well as the creator for imminent demise).

Open relationships CAN and DO work. However, it requires absolute mutual trust, safety, and maturity. Without those, keep your fantasies where they belong...in mind.

"the only problem i see is with her, not with me".
I so have to disagree with this statement of yours. You seem to have this backward.
"ive never cheated. i never will"
That statement alone clearly says where you ARE presently with your gf, yet, suddenly, when confronted with this sexual invite, your intentions have clearly stated that it is you who is changing the path you both, (you and gf), have already committed yourself to.

It's true that anything's possible. The fact that if you were to approach your gf and ask her if she'd be interested in trying an "open" relationship, and she'd be willing ...IS possible, (but you're asking her this....why, again? For who's benefit, exactly?).

From the sounds of your descriptions of her, "clingy", "testy", "flip out" (therefore undoubtedly, hella jealous), the chances of obtaining an "open" anything is highly improbable.
And just from your conduct alone in this post...It's more than apparent that you are not approaching this with your gf in mind, whatsoever, (this is clearly about YOUR hormones in high gear), therefore obviously being one-sided, quite selfish, (nothing mutual about this), and definitely a disaster waiting to happen.

And let's just say that your gf IS good with the open idea. You have to ask yourself....Are YOU prepared to accept the fact that she wants to "bone" or "bang" other men?
After all.....this "open" relationship that you would like to have (however convenient it would be for you at the time), means SHE gets her's too, you know. (Open = Mutual?) This "open" isn't just about YOUR lust....She may have some of her own that you aren't even aware of, (let alone capable of dealing with).

Be careful for what you wish/lust for. You just may get it.

**I have to apologize for my bluntness. I don't mean to offend. It's just, there's no beating around the bush here, (no pun intended), due to the fact that this post was quite blunt and offensive to many in its own right, which, in my opinion, has earned the same in return**

Shangrala