To be honest, math is important for us as it helps train our brains to think quickly.

Surely you need.

These kind of knowledge is not about whom you're going to become but as some sort of the basic fundamental knowledge for learning and understanding the world itself. 

But on the other hand, whether those information matters to you or not depends on you own. 

I don't see any connection between these two different things. One is about math. Another one is about designer.

If you want to be a designer, then go ahead to get it and focus on the relatives.

For me, math is difficult. So I choose to learn languages.

To be an ordinary fashion designer, of course, it's not vital to not master complicated math knowledge. But to be an excellent fashion designer, it might be very good and necessary if you have a certain extent of backdrop of mathematics.

As a natural science, actually, math is so essential and basic. It's been widely used everywhere in modern life. Don't you ever hear the story about those famous architects whose invaluable masterpieces passed down are in general the crystal of their creativity and mathematics?  

As the old saying goes, every road leads to Rome. Even for people whose jobs have nothing to do with design and have everything to do with literature or communication, you don't know on which day you will have to use math. 

In short, for those mathematics in your text book, it's reasonable to exist, and there is no excuse for you not to learn it.

Back in high school and even at the university, I was not very good at math, either. My math teacher once told me that learning math or solving the math problems was actually helping you develop your brain and gain thinking ability. In that case, learning math can be helpful. Maybe you can't find any connection between a fashion designer and math now, you can still be benefit from math in some other ways in the future. Just try to learn it and don't be too anxious.