It sounds as though you are afraid of you may get injured or hurt, and to be honest I can't imagine that happening if your instructor knows what he or she is doing.
For what this is worth...
I will never forget my first boxing sparring session. I was hammering the punch bag, had just bought my gum shield, and my coach had told me how good my shots were looking in practice.
I got in the ring and imagined how impressed the gym would be as I danced round my sparring partner. So off we went, I pretty much ran to the centre of the ring to meet my opponent. Then it happened: BOOM! He caught me right on my temple with a big right hand. He racked me and my legs did a little dance. I panicked. I began retreating, switching stances, doing anything I could to not get hit again. I managed to last 2 rounds. I realized after that although I could punch well, I had a poor defence.
It was many months before I sparred again. In the interim I had learned to slip, duck and weave, block with my gloves etc. Next sparring session, I was doing ok, and then it happened again. BOOM! I got caught again, and it hurt. But to my own surprise, I didn't panic. I realized that yes, this hurt, but it wasn't the end of the world. I could take it.
This is just my experience. What I found was that, like most things, I was afraid of the ghosts and shadows in my head. The thing I was afraid of was getting hurt and not knowing what to do. But that wasn't my physical reality, it just wasn't the case. So that is what I meant by ghosts and shadows: my fears where like most people's, like yours. They were based on my perception of myself, and not reality. Yes, I did get hit and hurt. But when I realized that I could cope, my fear disappeared. I knew afterwards that what I was afraid of was a falsehood.
If you are afraid of getting hurt, don't be. What you have to realize is that fear is an emotional state. It isn't rational, it isn't logical. If you want help overcoming fear, two suggestions:
1) Talk to your instructor. Practice with them in a gradual way; get comfortable with them and them attacking you.
2) Work on your mind. There are a lot of wonderful teachings in martial arts that help you deal with the emotional and mental side of physical confrontation (amongst other things). Things such as meditation, self-hypnosis, breathing techniques, chi/ki cultivation...
The list goes on. I find these things can really help. Remember, martial arts can develop your physical abilities, but they can also develop your mind and spirit too.
I will leave you with this quote. It just goes to show you, everyone can feel afraid, but everyone can conqueror it too.
Sorry for the large post, hopefully of some use to you.
" You've got to have confidence and believe in yourself. At the end of the day you've got to look at your opponent and realise that he's as scared and as nervous as you, that's the thing you've got to bear in mind. No matter how bad you feel your opponent feels the same".
Ricky Hatton, Light-Welterweight boxing champion of the world, The Ring Magazines 'Fighter of the Year' 2005 in an interview last month in regard to fear.
"Let your food be your medicine, and your medicine be your food"