Check Field Response Before Posting Comment

So I decided to create a theme from scratch for my own personal use, and I have to say it’s been quite the learning experience. I’m committed to not using any plugins so that I can fully understand how everything works, but I’ve run into issues when implementing my own captcha.

I created an image with a code variable of $_SESSION[‘secure_code’] and then created an extra input box in my comment field in which the user is supposed to retype the code produced by the image. Everything works great, except I can’t figure out how to verify that the code is correct before posting the comment. I know that I have to deal with functions.php and probably change how WordPress handles comments, but I just can’t figure out how to handle this.

My code is as follows:

[Code moderated as per the Forum Rules. Please use the pastebin]