Hang in there! We are our own worst critics. It's probably not as bad as you are making it out to be.
If you really want to improve, I agree that taking a beginning quilting class is a great way to go, even if you have been quilting for a while. You will understand what the instructor is saying and can ask good questions.
Other than that, remember that there are 3 things that will help with your precision:
Cut accurately. Cut the whole project with the same ruler, or at least the same brand of ruler.
Check your seam allowance. Good idea to recheck if you use a different fabric or thread from the last time you checked as your needle position may need to be changed:
http://www.quiltingboard.com/tutoria...ce-t89997.html
Press carefully. Even though I've sewn for years, I still have to watch that I'm not pressing a tiny fold at the seam line!
OK. Maybe there are 4 things: as you make your sub units (HSTs, flying geese etc.) measure these as you go. IF the sub units are the wrong size, your block won't go together.