I can give you a girl's point of view on this. Hopefully some guys will post on here, and give more answers.
First off, I would like to say that you're putting too much stress on yourself. You said that you get nervous, and that's when you cannot get a hard on. Stress & nerves will do that every time.
Sex is supposed to be an enjoyable experience. I understand that this is new, and understandably you're going to have concerns, but generally you're going about this the wrong way.
To get yourself more comfortable, you should take it slow. Pleasure her, and take your time. This is a getting to know each other time for both of you. Find out where she likes to be kissed, rubbed, etc..
But then there also has to be time to pleasure you as well. This is a partnership, & if you don't feel like you're getting equal time, then you need to seriously sit her down, and ask her what's going on?
She may know that you're really nervous, and may not want to make it worse for you. I don't know. But She needs to get some satisfaction from this partnership, and so do you. You both deserve it.
So just communicate your needs, listen to hers, and take it slow. This is a wonderful experience of getting to know each other. Stop, & savour the moment! And relax...