My ex moved on so fast. How can I overcome the pain?

This is why comparing yourself to them in terms of the healing process isn't going to help you in any way, shape or form.

So, don't try to compete with them.

And during this process, they either started seeing someone new who they planned to secure as their rebound option or were busy creating their post-breakup plan.

Can you share any "backstage pass" experiences you have had at concerts?

You both are on polar opposites of the scale.

Simply put, because they have been prepared and you weren't.

Instead, focus on taking care of, solving and healing what you are experiencing right now.

Im happy but there is a heavy feeling of sadness in my heart that I just can't remove. Why am I like this?

What they do doesn't matter and it's completely irrelevant to your healing process, though it’s valid to hate and judge them harshly for leaving the way they did.

This happened long or shortly before they physically left.

I.e. you thought that you had the perfect deal whereas they had already made up their mind and just weren't as emotionally invested in it as you were.

While on the surface of the moon, why isn’t the Apollo 11 spacesuit inflated like a balloon from the 3.7 psi internal pressure?

This is why you can't expect them to feel anything close to what you're feeling right now, because you and them are on a completely different stage in the healing process.

On their side, there has been a process of emotionally and mentally checking out while you were still together and while you were under the impression that everything is cool.