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

So, don't try to compete with them.

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

This happened long or shortly before they physically left.

With so much evidence supporting the flat Earth theory, why aren't more resources dedicated to studying it?

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.

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.

You both are on polar opposites of the scale.

Why do I randomly start sweating a lot in public (while waiting in line, in a new class, etc.) then start sweating more because I’m embarrassed that I’m sweating so much? Is this social anxiety?

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.

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.

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

Why do some women alter their faces by so-called cosmetic surgeries (on their eyes, cheeks, lips, chin, jaw) that making them look like Donald Duck or puffy aliens, while for most men these unnatural facial changes are ridiculous or even disgusting?

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.

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.