I think it's fine to have a slight grudge against a former friend from school. What do you think?
It can be hard to let go of things, but I think it's important to do so. Why do you have this grudge?
I hate him so much because of how he treated me.
This sounds like more than a slight grudge. I understand that the two of you had a falling out, but keeping this grudge isn't healthy. Either make peace with him, or just move on and stop letting the past bother you so much.
I'd rather be miserable and focused on the past than happy.
It can be difficult to let go of things that happened in the past, but it's not healthy to dwell on it so much. There's plenty of things in your present life and in the future that you should focus on to make you happy. Try to forget about the past and focus on the people who make you happy now.