The thing I found helps most is once your square is sewn, make sure and square them up to the "unfinished" size, before sewing the other squares together or the sashing on. It doesn't seem to me, just by looking, that you've done this before sewing the rest together. I used to have your same problem when I was new at it, had a more advanced quilter show me that, and I do it faithfully since. You'd be surprised how even slivers cut off really works. Just a suggestion. Don't get discouraged, hang in there...it gets better!