I feel your pain! I am working on a half triangle piece to show off some hourglasses and although each square individually was almost perfect (see, almost) once I began sewing my rows together it began to look like a hot mess. Of course, any person not familiar with quilting has looked at it and ooohed and awwwed over it but when I look at it the seams just taunt me!! I have found the best method for this particular project of mine is to pin where the seams nest and to hold the fabric together firmly while I'm sewing, not so firmly that it warps but just that it behaves and then right when I get to that nesting point I take the pins out and sew right over and it does the trick. That has worked so far and then when I press it it all looks much better. Good luck, I am a beginner like you and I think we just need to keep practicing and quilting and quilting and quilting.