As Kiera said, without knowing more specifics (are you afraid that you won't please her? that she'll reject you for some physical reason?) it's really difficult to give good advice. That said, talk to a therapist or psychiatrist. Phobias are all similar in one respect--an irrational fear or something or someone or someplace--meaning that the treatment for one isn't a lot different from the treatment for another. Good luck.
Want to spend some time wallowing in a Recommended Read? Pick one! Or two! Or seven!
whatever your scared of, take small steps. Kissing, hugging, holding her. Just go with what you find comfortable. Take things slow. Depending on how old you are, you may not be ready for sex. Practise on yourself, picture a girl is doing it. Maybe that will help?
Yeah, we really need more information. Is your phobia about performance? That you won't satisfy her, or that you won't know what to do? Are you phobic about showing your body, or showing emotion? Or confused about what girls want and expect? Each problem has its own solution.
I'd like to add to Hera's post, having gone to various psychiatrists and psychologists for depression for more years than I like to think of. Look into some LCSW's too (Licensed Clinical Social Worker). They're more active and engaged in a therapy and less academic and dogmatic than the PhD's. I'm making more progress with my LCSW now than I did in like 25 years of conventional therapy/analysis. I also picked a female (I'm a male) because so many of my issues involve women.
Blue berry does not work......trust me. Strawberry jelly is the only thing that does.
My Karma just ran over your Dogma