Now note, I'm just some guy from the Internet.
I don't know you, or your girlfriend so you may very well be best off ignoring me completely.
Anything I say is strictly based on the information I've been told here, so please keep that in mind.
That being said, I can relate.
Love is perhaps the strongest, most eye opening and most wonderful feeling that human beings are capable of possessing.
But it is also the most fragile, blinding, and painful feeling that human beings are capable of possessing.
And quite honestly? You can never control who you love, love just doesn't work that way.
But you can choose how you let those you do love affect you.
Loving a person isn't the same as being happy with them, you may gain happiness from them simply by loving them. But that doesn't mean there may not be pain that outweighs it. And on top of that, her cheating on you shows the dedication/loyalty to you is not on par to what you have towards her. Especially if she's done it more than once.
And in such a situation they simply are not right for you. A relationship should be people who love, care about, respect and make each other happy. Not one person being in constant pain/neglect from what the other is doing. And I won't lie, ending such a thing hurts. You don't instantly walk away feeling happy, in fact you're likely to walk away even more hurt and broken at first. But what walking away from such a relationship does it prevents the pain/grief from continuing to get worse, and it will start a healing process which by the end of you should be happy on your own again, and be able to find someone who truly does care for you, and wouldn't take advantage of you.
This was something that had to happen near the end of my High School career. It hurts, a lot. And the love honestly doesn't go away, you will still feel moments of pain and regret. But ultimately you learn to be happier, you become the source of your happiness. You walk away stronger for it, and will one day find someone who get along better with, where drama like this shouldn't happen. Where instead of coming online and writing posts about how hurt you are, you come on and brag about how much they make you glow, feel special, happy, how amazing they are etc.