Why does the pain of breakup comes in phases for me?

Like I'll spend some hours feeling fine without thinking about her and even if I think about her I don't really feel the pain.
But some hours later I'll be feeling like I won't survive if don't hear her voice again because of how intense the pain is to me, I'll cry out my eyes.
And then it will pass and I'm okay again. It's only been 5 days since the breakup, and whenever I pain comes it comes with regret because I was the one who suggested the breakup. I keep thinking if I made a mistake but deep down I know it is the right decision.
Why does the pain of breakup comes in phases for me?
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