I'm still not over him and I feel like I made a mistake.
After all this time you're still not over him? Have you thought of a solution to this problem?
My solution is to re-evaluate my past relationship with that man.
It's okay to not feel like you can completely move past an old relationship.  However, it is good that you are re-evaluating your past.  You'll probably find that things weren't working out for both of you.
No, I'm sad because things went downhill for me, so I sort of feel like the break up was maybe my fault.
It's good that you can reevaluate your behavior in the relationship, but that doesn't mean there wasn't a reason for things to end. Are you sure you're missing him precisely or are you maybe just missing the normality of having him in your life?