I agree with elisabrat......dont give up. Keep trying. When I was working on blocks for another swap they kept coming up 12 1/4, went back and looked at my seams and they were about 3/8 instead of 1/4, so I adjusted my needle position to get scant 1/4, and today the blocks I was making have come out exactly 12 1/2. I am so excited.
The other thing is what how you could be pressing your seams. I would start with some simpler blocks and again use your ugly fabric for practice. Keep trying it will happen, and come out right.