How to fix a crochet balnket on the ends of the rows?

I am working on a blanket that I am crocheting just doing a double crochet. well I started off fine but as I am working up i am finding that I am getting smaller on each row. I don't want to tear it out because it is different colors and i have already cut the yarn when changing colors. Is there a way to fix it? I was thinking maybe a border around the whole blanket would maybe fix the problem. Help!