First I want to say, stick with this wonderful craft. Don't let these little problems hold you back. It does get better! The old saying "Practice makes perfect", works only as long as you practice and are getting the same results every time.
I was making wonky blocks and it was the result of 3 different things for me.
First, I was tailing off when I got to the end of each seam, because I quit looking at it. I now use something to stabilize the seam all the way through. I usually use my left hand and a pin if I am sewing over a lot of seams. I have 2 sewing machines and the little Runway Brother is a lot worse at tailing off than the Big Brother Embroidery/Sewing machine combination. I had such problems with the tailing off that I thought about buying one of the Pfaff's with the built in walking foot that is supposed to keep the seams even, but just never got around to it, and didn't budget for it. Instead, I figured out where I had to move the needles on each of my machine to make that "perfect" first block of any project and then continue to use that setting. And, yes, it is different on both machines. I, also, use a 50wt thread for all my piecing. It really does help.
Second - One of the other things I was doing was watching my needle, instead of watching the edge of the fabric which was my guide line for where I was supposed to be sewing. I often wonder how many people do that and never manage to get a 1/4" seam. Training myself to watch that fabric edge has increased my accuracy.
Third - I was pressing with Steam. I had great flat seams but the steam distorted my fabric. Turning off the steam meant my seam wasn't quite as flat, which I found doesn't usually matter for my simple piecing, and was very helpful to keep my fabric from going wonky. "Love that word." Everyone is so right when they say it isn't the size of the seam, but the resultant block that matters.
I bought some precut kits that had to be sewn perfectly for everything to fit together. That really made me get familiar with making a 1/4" seam that worked. I ripped/unsewed a bunch but it made me a better stitcher. I still lose some points every once in a while. It does aggravate me, but I get over it.
If all else fails, take up paper piecing. You sew onto a piece of paper that has everything marked for you. The results are frequently stunning. Carol Doak has a Yahoo group and she shares a lot of free patterns. Doing the paper piecing has also made me be more careful with my stitching lines.