Why does it hurt so much if I don't want to be together anymore?

I was writing a Xmas card for my ex and ended up writing 2 sheets of paper instead and going through a half a box of tissues crying instead. We've been together for 6.5 years. We've been apart for almost 2 yrs now! I've been in another relationship and in love since then (and a breakup), but I could never let go of my relationship of 6.5 years. I think I'm afraid to let go of it. I think I'm afraid to process this pain, and most of the time I choose to hide it in the dark corner of my mind room, pretend it's not there, like a monster I'm too afraid to face.

When we broke up, I didn't know who to talk to. No one in my cirlce has been through a divorce. I thought that people who haven't been through this kind of pain won't understand. I found like 5 people, but they all realized their partners treated them poorly, or they were too different, therefore they broke up. My ex is the best human being I've ever known. He's kind, smart, reliable, trustworthy, honest, humble. I call him noble. I was so lucky to have met him. Yet, we've fallen into routine and boredom of a relationship, but it was our first serious one, and we didn't have the skills and knowledge to know it's normal, and how to preserve what we had. We wanted novelty and excitement. We said: life's too short, and we want to fall in love one more time, and we said goodbyes. It was a mutual decision.

These past two years have been very difficult, for both of us. I wish I knew how to glue it back together but I don't. He doesn't know either. We met up in November for a few days, and nope, the reality is that it's too complex. I wish it was that easy. Humans are flawed, and our brains and feelings are tricky. We decided to move on. It was a mutual decision again.

But why am I crying so hard? Why am I wailing? Why am I crying on the floor? Why am I going through a box of tissues at a time? Why is it so painful and so hard? Once again, I don't have anyone who's been through that who I could talk to. 7 cups, help. Why is it so hard? What's next?

I think I grieve the loss. The loss of all the good that we shared. How good he was to me. My fear that I won't meet anyone as good as him. There aren't many people like him. I haven't met them. What is it? This pain. It's so strong I wail and I can't understand it. Please help if you've been there.