Just out of relationship I wanted to break up. So why so hard to let go?

Obviously, you still feel for the person.
I cannot say what or why, exactly, because you haven't provided us with much information, but you miss some part of your relationship.
If you broke up with him already, it is too late (re-tries at dating never work, except in special circumstances)

If you would like more personalized answers, could you please give us more information, such as age, reason you broke up, how long you were together, etc.

Even if you want to break up with someone, its still hard. You generally still care about them in some way... maybe even just as a friend. So its hard to know that you hurt the other person. Also, it was a failed relationship. No one likes to fail at anything, especially something that they have invested a lot into. It takes time, just try to focus on yourself for a while and focus on moving on.

We could use more information. But every breakup is difficult.

Furthermore, the deeper connected you were with the other person, the more difficult and longer it is to heal from the breakup.

Think about it this way, if it wasn't that difficult, then you weren't really into that person in the first place.