While having sex and masturbating fantasises range from general to extreme and sometimes bad as well. There is nothing to feel guilty about unless you fantasize the forbidden. But the forbidden makes it more pleasurable is what is said in the world. So to give you a solid answer, you aren’t the only guy with this problem you find peculiar.
The answer to your questions:
No matter how cool a person stays while not having sex or while masturbating, something’s cannot be controlled when having sex/masturbating. It is no disease or an issue. Its matter of time one overcomes this. In spite of the guilt if you’re doing it over and over, try imagining an other girl. Unless you don’t misbehave with your friend in the real life, no issues but try getting your mind off her while masturbating, because it gets pretty hard when you talk to her because you get all your masturbating thoughts when you’re around her, so make a beginning by thinking about some other stuff.
It is no wrong to fantasize. If you’re very much attracted, pop the question and ask her for a date and see if you can make your fantasies real. That is a good chance. If she is already engaged, well limit your fantasies and divert your fantasies off her if she is with someone.
No, you’re not alone. There is a long line and you’re just an other guy to be thinking this way. Don’t worry, you’re good, but as I said, take care and try diverting.