Well, masturbation is a very stress releasing exercise so it has its benefits. Because you're getting nervous, maybe you're having second thoughts about doing it or you're getting frustrated. The biggest problem with concentration could be the environment. Get to a place where you feel most comfortable and have no risk of intrusion. This has the biggest potential to fix your problem. Sometimes a bedroom will get you nervous because someone can easily walk in. Long periods in the bathroom can get you nervous because you might be thinking maybe some people know what you're doing or curious about it. You should know that girls like it different ways. There is no set way to get a maximum experience. However, you can try different things. As far as fingers go, you have to find the ways yourself. Try different things. Some things hurt one girl and pleasure another. See what I'm saying? The most basic technique is to massage the clitoris. This can be done circular motions. So try different things and see what your comfortable with.
Here is a site to help you further. It can explain each way to masturbate. And it has different techniques to use fingers.
Click Me
Good luck,
~jamesish~