I think most of us share this fear in some way, especially when things seem to be going well and what is in your head is being converted to reality so nicely. We all know that problems will inevitably happen, and we will have to deal with these problems - maybe not so successfully - and that will spoil the great feeling of everything going so well. I think this fear may come from some insecurity about our skills: we were lucky enough to get it right once but maybe never again. I think the way to overcome this fear is to remind yourself that you have the talents to correct any problems that may come along - and if you don't know how to do it, someone here at pF probably knows, if you ask.
A great figure starts with a great sculpt. Paint away fearlessly, and I think you will be rewarded. Also remember that even if what is in your head does not get fully translated into reality, it can still be a great figure.
Good luck.