I´m sorry that you guys broke up and i hope that you will find someone who can counsel you on how to get your green card.
I think you have a great chance working in the US and if you like it you should try your best to stay instead of running back home because things didn´t workout as you planned and you´re tired of doing it the hard way.

The thing is i also hope that you will understand your ex girlfriends reaction at some point. No one likes ultimatums and if you love a person you don´t put the pressure on them like that.
Just because you would sacrifice everything doesn´t mean that others have to do the same to prove you they love you.
Everyone is free to make their own choices – don´t compare your relationship with that of your friends. Don´t rely on someone else to do something when you can do it yourself. I believe she loved you too because even though you were badly paid and there was a danger that you might leave – she still stayed by your side.
I´m telling you this because i did marry for that reason a few years back and my husband became violent and cheated on me once he “secured” his card. I was young and idealistic and maybe naive when i gave in to him pressuring me. There´s many negative examples and your girlfriend made the choice not to take the risk if she wasn´t yet ready for marriage. And that´s totally okay and I wish i had done the same thing back then. Still i wish you the best.