Concerning Victor's refusal to make a female in Shelley's Frankenstein, actually, Victor eventually agrees to make a female.  At first he refuses, but then he agrees.  He travels to England to learn from experts there, then travels to Scotland and finds an isolated area to work.  He has the female partially built, when

I trembled, and my heart failed within me; when, on looking up, I saw, by the light of the moon, the daemon at the casement.

The "monster" surprises Victor at the door of the cottage.  Victor has a knee-jerk reaction to the look on the monster's face:

As I looked on him, his countenance expressed the utmost extent of malice and treachery.  I thought with a sensation of madness on my promise of creating another like to him, and trembling with passion, tore to pieces the thing on which I was engaged.

So his "refusal" to make a female is actually a passion-filled gut reaction to the monster's facial expression.  It's not reasoned out, but, instead, is an emotional reaction to the monster's surprise presence at his door.

To find the answer for this question, you can look in Chapter 20.  Victor gives a number of reasons there.

He worries about making another monster because he wonders if the female monster will be just as bad as or even worse than the male monster.

The male monster had promised to leave Europe but the female hadn't.  What if she refused.

What if seeing the female monster made the male more aware of his deformity and more angry?

What if she doesn't love him?  Won't that make him angrier?

What if they reproduce and make a whole race of monsters?

Why does Victor refuse to build the creature a wife?

In the book, Frankenstein, Victor considered making a fellow creature so that the creature might not be alone. After hearing the creature’s speech about being companionless, Victor considers creating a fellow being for the creature. However, Victor soon rejects this idea after examining the possible repercussions. 

First, Victor is afraid that the new female creature might reject the original creature or his ideas. While Victor is thinking, he becomes afraid that the new creation might despise the old creature or might reject the contract that the old creature proposed (of them moving into the wilderness and living alone). If this rejection occurred, Victor feared the consequences might cause disastrous results for his creation, especially since he experienced so much rejection and solitude in his life already.

Subsequently, Victor also feared what might result after creating the new female. While considering the notion, Victor realized that even if the new female decided to go into the wilderness and accept the male creature, there could still be severe consequences. For example, the two creatures could give birth to even more of the creatures. As a result, all of humanity might have to encounter the new species. Victor states:

I shuddered to think that future ages might curse me as their pest, whose selfishness had not hesitated to buy its own peace at the price, perhaps, of the existence of the whole human race.

Lastly, Victor refused to create a companion for the creature because of his belief about who the creature was. As Victor looked at the creature, he saw a monster with malicious intent. As a result, he was afraid of what the creature might do, especially if there were more of his kind. Thus, he “tore to pieces the thing [fellow creature]” he was creating for the monster and refused to create a fellow companion for him.

Consequently, Victor considered multiple reasons for his refusal in creating the companion. However, the one that finally led to his total rejection was his beliefs about the creature. Seeing the creature as a “monster,” caused him to reject the creature’s proposal and forced the creature to live alone.

Why is it so difficult for Victor to decide to make a female creature?

In Mary Shelley's Frankenstein, Victor is obsessed with creating a creature. His motivation is sound: he wants to stop death in its tracks. But he plays God, for no man should be able to create life, and when he realizes what he has done, he is horrified. The creature is monstrous in Victor's eyes: where he thought he was creating something "beautiful" with white teeth and black flowing hair, he realizes he has created an abomination to man and God. He runs away, never assuming responsibility for the creature he has created.

Being rejected by Victor, the creature wanders the earth, learning about nature, how to live, and the cruelty of people. The creature becomes very lonely. When he meets young William Frankenstein, he believes a child cannot at such a young age feel discrimination against one such as the creature for he has not learned this kind of hatred and rejection.

William, however, becomes hysterical and struggles to get away. When he uses his father's name to persuade the creature to release him, the creature hears Victor's last name, and decides to punish Victor by killing his youngest brother.

Once the creature does this, he also arranges for the death of Justine. Victor hates the creature and wants him dead, which is unrealistic with the monster's speed and strength. When the creature gets Victor to sit and listen to his side of the story, Victor begins to feel responsibility for what he has done to his creation. He finally agrees to create a mate for the monster, who assures Victor that once he has a companion, they will disappear forever.

Victor goes about traveling to other countries to speak to those who can help him, while buying and collecting what he needs. However, he begins to worry that if the monster has a mate, they could create twice as much havoc as the monster has already done. And at the last minute, disgusted by what he is doing (with eyes wide open as they had not been the first time), Victor decides he cannot create the monster's mate, which spells destruction for others that Victor loves.