I gotta be honest with you. I do not think you made a mistake. If you told Dev to break up with her girlfriend, then it would be a relationship based off of something that she left up to you as far as making a decision. What you did was let her off any hook and allowed her to make the decision herself. The fact is that when she had the choice between you and the other, she chose the other. If you made that decision for her, then that would have been an elephant in the room for your relationship somewhere down the line. I know that the outcome didn't turn out the way you hoped it would, but she had to be the one to decide to break up with someone. Who knows. If the choice was so hard that she wanted you to make the decision, maybe she might decide she made the wrong one and come back to you. But that has to be her decision. I don't think you should regret your choice. But if you must, maybe let Dev know how you feel. Don't tell her to break up with the other person, and don't tell her not to. Just tell Dev that you have these feelings and that when it comes to a romantic relationship, she can't have you tell her what she should do. 

warmaker : When she asked me about breaking up with her gf just to be with me, that was a decision I cannot make. that's why I ended it between us.  Dev needs to know what she wants and I hope in time she will figure it out.