How do I get the girl that loves me if she's in another relationship?

To explain a little farther, this girl and I have had feelings for one another for a while now. Almost 2 years. When she was single I was engaged to someone. Long story short, I got betrayed and this girl I'm talking about ended up in another relationship with someone. Now she's engaged. I finally started to hang out with her and her boyfriend about 2 weeks ago. Everything was fine. We were just all hanging out and then she tells me she still has feelings for me. I told her I do too but I didn't want to mess up her relationship. Everything kept going smoothly once more until last night. I became the other man in the relationship. Her boyfriend went to get her something and she reaches over and grabs my hand and we started holding hands. I was stupid and I leaned in and kissed her and now its something that's gonna be expected of me from now on. She wants to talk about it a lot now and she smiles at me when I drop her and her boyfriend back off at their house and she still wants me to hold her hand and all that and her boyfriend is COMPLETELY unaware of what's going on. It's difficult for me because I've never done anything like this before. He's really abusive and I see new scars on her every time I see her. So I honestly don't feel too bad about the whole thing. But still, I'd rather her just be with me than live a lie since this is apparently gonna go farther than it already has. Do I have the right to do this? And if so, how do I do it?