Insecurity has entered into your relationship, and it's destroying its very foundation: trust.
You say that you disliked his friend before ever meeting her. While it's normal to have a little twinge of jealousy when your boyfriend spends time with a female friend (even the healthiest of relationships deal with this at one point or another), it's extremely abnormal to immediately have the level of jealousy and distrust you did for his friend. Why? It suggests that somewhere deep down inside you believe your boyfriend could cheat on you. This is extremely dangerous to your relationship, because love is nothing without trust. If you trusted your boyfriend, you would have at least waited until you met his friend to get an idea of whether you truly need to worry about her.
The dangerous situation you've put yourself in: You and his friend are both important to him. The difference between you and her, though, is she's not demanding that he make a choice between you two. You are. Whereas she represents someone who has consistently been there for him and supported him in his decisions, you have come to represent someone who questions his fidelity, and will try to change him. In addition, your questions about whether he has ever had feelings for her may have made him wonder if there is something special and potentially romantic between them. Trust me, I know from experience...
In fact, I was in your shoes, once. I was in a very happy one-year relationship. Then, my ex boyfriend became good friends with a girl from his study group (college). I met her and initially everything seemed innocent. However, soon I started noticing that she seemed to be paying a lot of special attention to him. So, like you, I approached him about it. I expressed my concerns that she was getting a little too close for comfort. I asked if he had feelings for her, and he told me they were completely platonic. I tried to trust him and relax, but they continued to grow closer, so I continued to bring up my insecurity about it. Soon, his behavior changed, and he started sneaking around. I asked him to compromise with me, and at the very least stop texting her while with me. Even though he knew it was killing me, he wouldn't stop. When I broke up with him because I couldn't handle the betrayal (I found out he had started to cheat on me), he told me, "I didn't know I had feelings for her, until you kept bringing it up." True, his infidelity played a part in the destruction of our relationship, but it was really my insecurity and distrust that truly led to its demise. I inadvertently pushed him into the arms of another woman.
I'm afraid this is what is happening to you. The more you have mentioned your insecurities, the more he has shut off to you. He feels he's constantly being distrusted and forced to make a decision between two people who are important to him, when he has done nothing to break your trust.
IT'S NOT TOO LATE FOR YOU! If you want to make your relationship work, you HAVE to make it a point to get to know his friend. Sit down with your boyfriend and tell him that you think it would help you to be more comfortable with the situation, if you were to get to know her better; that you recognize the amount of worry about her got out of control because you were making judgments about her intentions without even really knowing her. This gesture will likely make your boyfriend very happy, because it will show that you are willing to give his friend a chance. Plus, the fact that you would seem to be making a compromise with him may make him more likely to compromise with you and willingly change a few things to make you more comfortable. Another thing you might consider is telling your boyfriend's friend (after a few hangouts) that you initially were nervous about her, because she had such a close relationship with him. She may then put your mind at ease by assuring you that you have nothing to worry about. Regardless, the more you get to know her the better things are going to get between you and your boyfriend. After all, if his friend likes you, you'll have one more person routing for your relationship.