I would like to make you aware of the concept of safety behaviors. Safety behaviors are ways in which we change our behavior so as to avoid or minimize anxiety. So for example a person might go to the food store in the early morning because a crowded store makes them anxious, even though their actual preference (aside from the anxiety) is to go at midday.

If one uses safety behaviors to avoid anxiety, one's world narrows and diminishes.

The better approach in the long run often is to face the anxiety. To go ahead and do what you want to do anyways, and tow the anxiety along if you will.

It's good that you go to the gym even though a nervous wreck. You go despite the anxiety, this is good. Over time you can start to feel better about the gym, especially if you talk with someone about what makes you so anxious about the gym.

If the anxiety is too much, though, one can use tools like breathing exercises and self-talk to get past the anxiety and still do what you really want to do.

So with the costumes: if you do not act opposite to the anxiety, you might never make another costume. Imagine how diminished your life would be then: to never make another costume for fear that it won't be as good as your previous or best costume. In my view, it's better to question, face, manage the anxiety and keep making the costumes! So for example: let's say that the next costume isn't as good as the last one. What does that mean about you? What bad things will happen?