Dear Mominaflash--
Wow. As hard as it is, and as much as you may want to punch me in the nose...this too shall pass. She will grow up, and every step she takes toward maturity will help.
Okay, while you still have your fist poised to poke me, don't give up on a relationship with your maybe-stepdaughter. If you do, your relationship with your boyfriend will be miserable. Eventually, you won't spend time talking because it's always about the daughter. The daughter and father will be pushed closer together, and while their being close doesn't threaten your relationship with the father...when the closeness is defend against you...it's just not a good way to go.
The first step is to calmly assess your relationhip with your boyfriend. If you marry him, he is a package deal. If you marry him you will have to rethink your approach with the daughter. No one has to take on a package deal. There's no shame in saying, "Sorry, but I don't want the complications of marrying someone with a child from another marriage..."
If you choose to stay, it's package deal all the way. I became a stepmother at 22, and while being young has disadvantages, there are advantages, too. You have energy and time you won't have in twenty years. The fact that your maybe stepchild is a girl is a help, too. There are 'girl things' that lend themselves to stepmother-stepdaughter enjoyment.
One last thing. When I became a stepmother, I first thought, 'this will be easy and fun.' Within a year I had feelings I didn't know existed in me. I felt jealousy, anger, and other feelings I didn't like and didn't feel good about. So,it's important to cut your self the biggest break in the world. This stuff is hard.
Also, I'd advise against using family to talk about these difficulties. If your boyfriend becomes your life partner, family will not forget the problems you've had with his parenting and not forget the problems your stepdaughter has caused you...and these memories will make it hard for your blended family to be comfortable. I do hope you can find a friend where you can 'just let'er rip' and say anything you want. The forum's a good place, too.
Good luck,
mysteryshrink