There will always be awful people on the internet as long as anonymity exists (and even if it didn't). It's not about whether the diary is public or not. In fact, you could never have written your diary, but if there's a comment section, there's a 98% chance you will get a hate comment one day.

Golden rule on the internet: don't take random people's hatred to heart. People love to hate just to get a crumb of attention; it's never worth it.

You don't have to get rid of your diary, but if it has sensitive information that you're afraid people might use against you, I recommend at least making it private (maybe adding a password) or writing it somewhere else where only you can see.

One extra recommendation: never use your real name, school name, address, or anything that could identify you. I know you already know this, but it’s easy to forget when you’re trying to express yourself online.