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

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

So, don't try to compete with them.

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.

Do you think some men have sex with prostitutes because they're too afraid to talk to women? Money does the talking for them.

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.

This happened long or shortly before they physically left.

My cantankerous beagle is very badly behaved at the dog park and always starts barking at the other dogs. Would pepper spray be an effective method to correct his inappropriate behavior?

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

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.

You both are on polar opposites of the scale.

It’s been over a month since I stopped taking sertraline but why do I still feel side effects like brain zaps and anxiety mood changes? The root cause of anxiety it’s your thinking and I perfectly master that better than before so it’s hard lately.

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.

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.