That may have supposed to have been seed stitch along the border, but it is not. You ended up with ribbing. Seed stitch alternates the knits over purls and the purls over knits on every row. Ribbing lines the knits over knits and purls over purls and that is what you have done.
I think you need to learn to do a mattress stitch seam. That will sew it together quite nicely. You may have to experiment a little, but you may be able to get it to look pretty much like it was knit closed to start with if you do it just right. Here is a link
to a site that has some good pictures of how to do it. (But it is not for ribbing) If you hate sewing don't get too worked up. Sewing in knitting is very different than sewing fabric.
I have been working on a sample of how to sew up ribbing. You need to draw together a half of a knit stitch from each side. I am trying to get some pictures ready for you to look at, but am having trouble. I am going to go ahead and post this for now.
Do you have the same number of rows on each front, but the looseness just made one longer than the other? Or do you have more rows on that one side?