The first nervous grumbling is, "I just can't write. I've been taking English for years, and I even got good grades a few times, but writing is still a mystery. Some people are born knowing how to write, but I was born untalented, I guess." Nonsense. To be sure, some people write with greater grace and ease than others, but not because they were born that way. The chances are that they write with grace and ease now in part because their parents read to them when they were very young and in part because as they grew up they spent many hours reading. Just as it is easier to learn French at a young age when surrounded by French-speaking people, so it is easier to learn to write if, at a young age, we read, and read, and read. Still, many men and women have learned French by studying it in college, and a far greater number of men and women have learned to write good, solid English prose in college.
With hard work, concentration, and receptivity to criticism, you can learn to be a good writer. You will discover, of course, that when you have something that you want to write about, something you need to communicate to a real audience, you will automatically be a better writer than if you are writing exercises and examples just to fill an academic course requirement. Within the context of a college course, however, you can learn the basics of effective writing. The second nervous grumbling is, "I don't have a big enough vocabulary to write well." Nonsense. You have a big enough vocabulary to read the daily newspaper and college-level math, science, and history textbooks. If you did not, you would never have been admitted to college in the first place. If your vocabulary is big enough for those activities, it is big enough for you to write well.
Good writing does not depend on a big vocabulary. Indeed, a vocabulary that is too big can be a greater impediment to effective communication than one that is too small. Clear communication is often impeded by big words, because by using such words you run the risk that your audience will not know what the words mean. The amazing thing about your vocabulary is that it develops along with you. As you expand your interests to include subjects requiring a larger vocabulary, you automatically develop the vocabulary to communicate about those subjects. If your Uncle Harry gave you a thesaurus for a high school graduation present so you could use it in college, I advise you to throw it away. It will only get you into trouble. You already know the words you need to write well what you have to say.