Jason,
I am in the same shoes as you. You sound like a nice guy and someone that treated his ex pretty well. The thing is, it's so darn easy to start falling in love with someone. I know this because I can relate to you. When I start loving a girl, I give her my all. Just like you, I did so many things for my ex girlfriend. I bought her most of the things she wanted, nice/expensive places to dine at, and just spent the majority of my free time with her. In other words, she was my world and I loved her so much.
One of the problems that people like us have, is that we tend to give them everything. We don't play any games of being "hard" to get. We get our exes whatever they ask, treat them nicely, etc. Then, they start to realize that they have use under the tips of their fingers. They know that whatever they ask, we'll simply comply. Not because we're whipped, but because we love them so much, and want to do whatever they like. It's hard to turn down someone, especially when you love them so much.
After about a month of the break up, my ex found a new boyfriend. She found someone completely different. She used to tell me that she'd never date X type of guys, but now she is. The point is, people change and their feelings change, like someone previously mentioned. They want to try new things and see how it is like on "greener pastures."
The thing I did to sort of stop thinking about my ex was to just ignore and block her. I didn't want anything associated with her since the thought of her made me mad and sad. I mean, how can someone who claims to love us so much move on so quickly, right? In order to block that pain, I simply blocked her and ignored her as much as possible. It's not easy, but I tried my best to mask the pain and sadness I was feeling. I hung out with friends and just vented out to them from time to time.
Time does really heal. It takes time, but slowly, you'll learn to let go. I think for you, it's good, since your girlfriend cheated on you previously, and yet she's acting like this. She is taking you for granted. You got to get out of her life. Don't contact her, nothing. Make her think about you and miss her. Let me remember all of the good time you spent with her, and make her regret and miss that.
It's ultimately up to you to decide if you still want to go for her or not, but make sure to make a clean mindset before deciding on anything. The guy she's with is probably a rebound. This means there's a high percentage that she'll come back to you. In the time being, do things to work on yourself. Do things you couldn't do when you were with your ex, such as hanging out with friends, working on hobbies, exercising, etc. Stop focusing on her, and focus on yourself.
It all comes down to this. If it's meant to be, fate will somehow work things out, no matter how futile it might see right now. If not, there's someone even better waiting for that right moment to meet you. It's hard to indulge all of this right now because you just broke up and feel pain and confusion, but as each day passes by, you'll start to realize that you CAN live without her, and that it's just a spur of the moment you're feeling sadness. Believe me, after every break up ( and I had plenty), I thought I could never move on. But after each on, I became a stronger person, and am perfectly fine. I do miss her, but I know it's time to move on, and not interfere with her life anymore. Good luck and don't be sad. It's not worth it. In no time, you'll be laughing over how stupid this whole thing was. Email me if you need someone to talk with and vent. Good luck, man and keep your head up high :)