Thanks for your advice, Sue and Markette. I started fixing mistakes on my own but it took so long (I was unraveling one stitch at a time because the thought of all those stitches off the needles was scary to me) Then I got halfway done with my square and realized I did almost a whole row wrong when I got to the end of it. I came here and read your replies, then watched the fixing mistakes video and got a lot braver. I unraveled the row and fixed it and was amazed at how easy it was. So thanks for encouragement -- I have a new skill!
By the way, I wanted to let you know that I've been using leftover yarn, so all those times I cut it I wasn't wasting good yarn. Now I realize I was wasting a lot of time, though!