The corolla is not equal to the Focus. How much will it cost you if you hate the corolla and miss the Focus and want to go back? I really miss my Focus when my wife takes it and I have to drive the mini van. The Corolla gets lower MPG too. In addition to being slow and obsolete.
I looked at trading in my Focus for a Fusion but the trade in value would be less than I owe. I'm sticking with my Focus, which runs perfectly and I am really happy with it anyway. Sometimes its cheaper and better to stay with what you've got. The used car market when I bought my Focus in 2011 was just insane. Stripped 3 year old econoboxes coming off lease with 40k miles for $13k. Hell no. The Focus is an awesome car. I'm keeping mine and you should just keep yours.