I understand how amazing he must seem and all you can focus on is the happiness that he brought you, but the point is he isn't making you feel that way anymore. You deserve to have someone that only wants you and KNOWS what they want. He should be entirely devoted to developing a relationship with you. This other woman is engaged. She doesn't respect her engagement, but more importantly (for you) he doesn't respect that she is engaged!!! He is being disrespectful to two women, and I'm certain she is just as confused as you are. She is potentially risking her engagement and you are risking your heart. I think you should let him go if he can't make a decision. I get that sometimes things happen and people get caught up in messy situations, but the way he's treating you is so hurtful and it plays with your mind. It must feel like you're on an emotional roller coaster, happy when he gives you the attention and affection you deserve and then you are miserable with confusion and fruitless hope when you're trying to talk to him or see him. He seems to have you in a spot where he knows your emotions are partially based on your relationship with him and that you'll tolerate him stringing you along and basically while he's behaving terribly. This relationship is also young at six months. If he has exposed this side of him, what more might you find? How long will you tolerate this? You have to be true to yourself and know that you can never change a man or wait for him to be better again. He might always toy with your emotions and he might never really be sure of his feelings for you, the other woman, or anyone else. He might never be certain. I think you should use your best judgment for how much more patience and time you will give him, but six months long gives you the opportunity to leave and realize that you want to be treated better and you know it will never change, rather than waiting three or five years before you realize, and then it will be so much harder because you'll feel you've invested way too much to "just" leave. I promise you deserve to be treated so much better and the fact that you're looking for advice from others really says that you know something is wrong and that you're unhappy. I recommend you try really hard to show yourself that you can leave him and be happy. Do you really want to spend years or maybe even your life with someone that didn't know how he felt about you, that didn't know where he wanted to be? You deserve so much more!!!
I agree with what Donna wrote about surrounding yourself with good friends and family. It's really important at this time to show yourself how full your life is without him and when you're not sitting around waiting for his call, his IM chat, or his company, you'll stop missing him. Six months can seem like a while with some people, as I'm sure it does with him. I know that if you didn't already have developed feelings and really adore him, you wouldn't put up with it. Sometimes feelings develop really fast and you might feel like you really care about him and he is too much to lose. In my relationship now, I honestly felt that right away and to have a break up at six months would have been absolutely devastating. If it is for you, take the time you need to grieve your feelings of loss. Relationships can bring so much happiness and to lose that feels so hopeless and grim. That's why you need to go out of your way to really enjoy yourself. It's hard and it really sucks. But one day, maybe soon, maybe longer, you will be fine! You will realize that in light of him mistreating you, you did what it took to treat yourself well. You will also have found the strength to demand what you deserve and with then doing things that you enjoy, you will be a happy, healthy and strong woman. This will put you in the best place possible for your next relationship. You'll be much more likely to attract men who want to treat you well and with the respect and honesty that you deserve.
One more thing- you also deserve to be a healthy-minded person. Being in this situation too long might make you needy and controlling in terms of dealing with a partner who has friends that are exes, it might kill your self esteem, and it might take you several years to recover the ability to have a healthy and trusting relationship. That's another reason why leaving him now might be really good for you because it can potentially be damaging to stay with him any longer. When you get into this mindset and the feelings of worry, jealousy, anger, hurt, etc become familiar to you as emotions of a relationship, it might be incredibly hard for you to recognize and then partake in a healthy relationship. Do what's right for yourself! It might seem really harsh, but not only will you be wasting your time, but you honestly could have difficulty in relationships for a very long time as a result of this one.
I hope you are okay and I'm sure this carries a lot of pain for you. It will definitely get better! I know how terrifying it can seem to leave the person you are with but I promise it will be better very soon. You have my support and I will look back to this post to see if you're written. I know it was about a month ago that you posted, but I really wanted to help.