I was the same way when I started out and I threw out a lot of really bad work. My best advice is to get good tools like a 12 1/2" square ruler, a 24" x 6" one, a good cutting mat and rotary cutter. Get some sandpaper dots to put on your ruler so it doesn't slide. I got some clear plastic sheets that you stick onto the back of rulers but not sure how well that does. Be VERY accurate when cutting, that is so important.
Be absolutely sure that you are sewing a 1/4" seam. Get a 1/4" foot if you don't have one or if your machine can move the needle to 1/4" that is even better. Go slow. Don't have to do a pedal to the metal speed. I don't go the fastest it can go and sometimes I even set my speed to the lowest setting (usually use the middle setting).
Depending on the block pattern I can tell if it's possible to cut my pieces a little larger than needed and then square up after sewing. I don't cut the squares for HSTs exactly at what it calls for but make it a little larger, 1/4" or 1/2" even, then square them up with a ruler and rotary cutter.
Use spray starch. It's amazing how much of a difference that makes. It's like sewing with un-washed fabric, but even better. The fabric isn't all soft and floppy.
Press intersecting seams in opposite directions and nest the seams together when joining pieces. Even if you have some difference in where they line up, put them together and pin. After you sew them together and they're locked like that, when you press them they will ease in together. Another little spritz of starch will solve any stubborn ones. I always press every seam.
That's about all the advice I can give. It just takes attention to detail and care at each step. I know that it's not always an easy thing to do, can be incredibly tedious, but when you've mastered that it really frees you because you're not fussing with every little thing.