I got much better after I started using a 1/4 inch foot. I never sew without it now. but still not always perfect. I also before sewing my blocks together I will trim them all up to the the same size.
I've always been using my 1/4 inch foot but found that some seams were still off. I've found that taking the time to test your 1/4 inch and then adjusting your stitching for that helps a lot.
Don't give up!