Dear Other Girl,
Wanting to be with someone and having something in the way is not an easy thing to have to do. Unfortunately, it seems to me that, as my mom always says to me, Nick wants to have his cake and eat it too.
He is Not Being Fair
It is not fair to you, or to his girlfriend, that he is messing around with both of you. I assume that his girlfriend doesn't know that you exist, which makes it even harder, because he really does have the best of both worlds and here you are stuck in the middle of everything.
I have a friend whose boyfriend of three months wound up having a girlfriend of over a year. My friend tried everything that you have tried; she talked to him, she wrote him letters, and she tried to make him choose, but all to no avail.
He is a Young Hormone Driven Guy
Basically he was, and Nick is, just a young hormone driven guy who really doesn't know what he wants but he is willing to take what he can get. If you are willing to allow him to be with you and this other girl then he will take advantage of the situation and he will see no reason to make a decision.
So, what am I saying? I'm saying that I would not advise that you allow this situation to go on any longer. While you are worried that giving Nick an ultimatum now will result in you getting hurt, you also have to consider how much more it will hurt as time passes and you become more and more attached. The longer you leave it to be dealt with, the harder dealing with it is going to be.
Also, remember that you are young and that you will have plenty of other "Nicks" in and out of your life in the future. You want to try and set a precedent for dealing with these situations in the right way so that you hurt as little as possible.
To answer your question- I don't know why he is bothering with you if he doesn't plan to break up with his girlfriend. Why not? If you allow him to take advantage then he is going to, whether or not it is the right thing to do.
Give Him the Ultimatum
My advice would be: Talk to him, make him choose, and deal with his decision.
You never know, maybe he will surprise you, and if he doesn't then it will just be a matter of time before you are able to learn from the situation and move on. Hopefully,you will move into a relationship where you will be the only girl and you will be treated like a princess.
Obviously you are not happy right now and the only way that is going to change is if you take action.