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.

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.

Do Europeans typically set heating to higher temperatures in the winter compared to Canadians and Americans if the temperature at a given moment is similar?

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

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.

Since the Brits can't steer their oil tanker, what makes them think they can take on Russia?

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.

So, don't try to compete with them.

You both are on polar opposites of the scale.

“From Garage to Glory”: Elon Musk’s Astounding Throwback After This 500th Falcon Rocket Launch Will Leave Space Enthusiasts in Awe - Rude Baguette

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

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.