Hello, I am not completely new to Buddhism but new to this forum. I have attended several retreats at Spirit Rock Meditation Center. I believe they are close to Theravadan, although the teachers (Jack Kornfield, Sylvia Boorstein, Joseph Goldstein, and so on) have adapted the teachings for the west.
I have been practicing metta meditation, and I just wanted to share this image I use when I get to the part about sending metta to my "enemy" (or as we might say, "difficult person.") So I usually start by having a good sense of metta for myself, then a neutral person. Then I have a difficult person in mind. I imagine two buttons in front of me. Pushing one causes harm to my difficult person, and pushing the other makes him a little happier than he is now. Neither button causes any effect to me, not directly. I reflect on the felt sense associated with pushing each button. I ask myself, "What would I choose?" I find that usually I really want to press the button that makes him happier. I start to realize it feels good psychologically, and of course it does me no harm. It does me no harm to wish happiness to another. I think the difficulty we sometimes have in wishing happiness for someone we dislike or envy is that we imagine it takes something away from us. But I just imagine those two buttons, and reflect on how pushing the button that makes him happier really has no negative side-effect. That's all it does. Make him happier.
Also, many teachers I've had mentioned that wishing happiness to your enemies makes a lot of sense, because if they were truly happy, they probably wouldn't be your enemy any more.