First, I'm very sorry you're here--no one should ever have to be, but it's a good place to be if you need it.
Your girlfriend is a cheater. She's been doing it since the beginning of your relationship.
Two years is a long time to be used. Two decades is a lot longer---and let me tell you, it pretty quickly turns to that.
Your girlfriend is looking for something to fill the holes in her soul. Maybe they were left by a father who was not there. Maybe she's created them herself. Either way, she's got a nice guy at home, taking care of her, but is out looking for creepy strangers to take care of her.
What void is she trying to fill? How does she think sugardaddy strangers will do that? Really, she needs some help that you can't give---just as the Craigslist (or whatever) sugardaddies can't.
Thing is, you also can't change her. You can't love her enough, buy her enough, shelter her enough, forgive her enough that she will love you in a healthy way.
She has to learn to do that herself.
She will need help.
And before she gets that help, and learns to love healthily, she will have to realize she NEEDS to do this, and then she will have to reach a point where she WANTS to do this.
Do you want to be your girlfriend's daddy? It sounds like she has a pretty sweet setup with you---you've got your five figures, and she's got the freedom to hook up with randoms from the internet.
Please consider that your relationship is not what you believed it to be. I know it hurts---trust me, I had 30+ years of a fraudulent relationship.
Also---if nothing else---GET TESTED FOR STDS. Do not have unprotected sex with your girlfriend. She is corresponding with internet "sugardaddies." They don't tend to take care of their "sugarbabies" without sexual involvement. (There can be exceptions, but not usually.)
You say she's got a good heart and is a good person. I'd say the evidence is to the contrary.
Let me recount a story (that my husband detests by this point because, well...it can be applied to him just as easily). My oldest nephew, we've come to learn, is a (diagnosed) sociopath. During his very late adolescence, he lived with my husband, me, and our kids. It was a volatile situation, during which I saw him for what he really was. My much-beloved nephew was a conscienceless criminal. A very charming, very handsome, very manipulative sociopath. I used to joke he was the only person who made it feel good to be walked on like a doormat. I don't joke about it any more because he got increasingly dangerous and destructive. Now, he doesn't even know where I live.
At the time, we lived far from "home." But soon after, we returned to our hometown, leaving said nephew behind---in jail.
So, I took my youngest to the pediatrician. This doctor was my doctor when I was young, and doctor to all of my nieces and nephews, as well as my oldest before we moved away. It was time for him to meet my youngest.
During the visit, he did the requisite asking about all the various family members. When he got to my nephew, I did a long song and dance about him, ending with, "But you know, despite all that, he's a really nice guy."
And the doctor looked at me as if I were insane and said, very plainly, "No he's not!"
And he was right. It WAS irrational to describe the things I had, then dismiss the reality with "But he's a nice guy."
He's NOT a nice guy.
Your girlfriend does NOT have a good heart. It might not be BAD, but it's not where it needs to be in order to be in a committed relationship.
It's time to erect some boundaries. It's perfectly reasonable to have requirements of the person with whom you share a bed and life, and to have consequences if those requirements are not met.
Don't let 2 years turn into 25.
ETA: Your girlfriend is an adult. She is doing the things she is doing because she chooses to do so, not because she had a lousy family of origin (FOO). Lots of us have screwed up FOOs and choose to behave as responsible adults. Until she knows she needs help--and WANTS that help--she will not develop good coping skills or better handle her past as she lives the present.
Most importantly YOU CAN'T MAKE IT BETTER FOR HER. You can't love her lousy father away.
She needs help. If she doesn't get it she will likely wreak escalating havoc in your life.
And that is something for which YOU must take responsibility--because the only person whose thoughts, feelings, and actions you can change is YOU.
Decide what you want in life. Do you want to play the role of Knight in Shining Armor (KISA, in these parts) or sugardaddy? To the bystander, it looks an awful lot like she's cast you in that role.
I hope I'm wrong.