You will become better at writing essays as you do so frequently through your class! Some tips that may help:
-A thesis statement is a beginning point for an essay. Since as essay is usually "written from an author's personal point of view," your first step for each essay you are assigned during your class will be to consider the general topic that has been assigned and decide what you think about it. That's your point of view about the subject, ready to be phrased as a thesis statement that you will "explain, comment on, or assess" in the remainder of your essay.
-I strongly encourage you to talk with your instructor about your concerns. You have a good understanding of the areas in which your writing is weak. If you ask for extra help in understanding how to improve your weak areas on one week's assignment and then try to use the same improvements on your own for the next couple essays, you should become able to use them comfortably on your own. Then ask for help with another weakness.
-Along with lots of writing, do as much reading as your class load allows. Be aware of the vocabulary being used as you read - use a dictionary to get definitions of words you don't know and think about how you could use them in your speech or writing. Notice how the grammar rules you have learned are applied in your reading and try using the same kinds of applications for yourself.
AP classes are designed to be demanding. This will not be an easy course for you, but if you are willing to put forth the effort and if you will ask for help when you need it, it will benefit you greatly in the rest of your schooling. Hang in there - and good luck!