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

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.

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.

So, don't try to compete with them.

What I have noticed is nearly every girl I try to connect with whom reject me are in their early 20s why is that the case?

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.

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.

This happened long or shortly before they physically left.

Trump says Xi agreed to restart flow of crucial minerals, but analysts say China won’t give up its ‘rare earth card’ - CNN

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.

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.

How can Democrats not feel hypocritical when they urge Trump not to be vengeful should be become president when the Democrats are trying to put Trump into prison?

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.

You both are on polar opposites of the scale.