Why do I always get flashbacks of bullying, even though it's been 5-6 years since I was last bullied?

It's something that has really affected you, even if quite some time has passed. I believe that bullying makes you stronger, you've been through it, and take comfort that you are no longer in that situation. Take it as it's your past and maybe try to talk to a friend or family member, bullying unfortunately is quite common, but the more time passes the better you'll feel. It brings anxiety because you fear it happening again, but you are doing well to acknowledge how it makes you feel, and many people have been through the same thing as you.

Because those memories that you where broken down by those kids sometime you see them and the memories come back

That can be because you're suffering a trauma from the time when you were. Maybe you're scared of it happening again, thus you're constantly reminded?

I believe that you have flash backs because once something bad happens it stays in your mind forever. Just think of happy thoughts and the future nit the past