In addition to the previous suggestions, have you pressed all the seams on the odd number of rows either right or left and the seams on all the even number of rows the opposite directions? This will allow you to line up those seams by "nesting" which makes it way easier to keep those seams straight. if you goggle "nesting seams" you should get some videos on how to do this. Once I got the seams lined up I pin them....then I look at the fabric between the pins along the seam line and if one side is a bit "baggy", I will pin additionally evening out the fullness along that seam....I usually start doing that by pinning in the middle and then in the middle of each half again....and yes...pressing those seams really helps and you can often feel if the seams have nested properly and I don't even pin them often anymore...but a good idea to start out that way.