what is victor's big fear in delaying his trip?

Victor has been worried about any delay in his trip because it would do nothing but keep him from completing the thing he had to do and didn't want to do...... the creation of the creatures mate. As you can see from the quote below; the necessity of the creature's completion leads him to separate himself from his friend. He has no joy, and no wish to be sociable, but he tells Clerval that after a short period of time in solitude he can return with a lighter heart and mind. His biggest fear is the knowledge that the creature's waiting for him; he's afraid to not finish the companion, and he's afraid of finishing. In other words, the man in being torn a part.

"We left Edinburgh in a week, passing through Coupar, St. Andrew's, and along the banks of the Tay, to Perth, where our friend expected us. But I was in no mood to laugh and talk with strangers, or enter into their feelings or plans with the good humour expected from a guest; and accordingly I told Clerval that I wished to make the tour of Scotland alone. "Do you," said I, "enjoy yourself, and let this be our rendezvous. I may be absent a month or two; but do not interfere with my motions, I entreat you: leave me to peace and solitude for a short time; and when I return, I hope it will be with a lighter heart, more congenial to your own temper."