I sew straighter when I chain piece (putting the next pieces of fabric to sew right after the one you're sewing, so that when you get to the end of one set, you go right to the next - if I haven't explained this well, google "chain piecing" for better explanations and pictures). It also saves a surprising amount of time.
I know it's discouraging, but sewing straight and accurately takes a lot of practice, like learning to play an instrument or paint or <insert skilled task here>. If you don't find it too tedious you can sew along the edge of some fabric a little, working on accuracy, or you can just make things, redo the worst seams, accept that it's not going to be your best work but that you will get better!