Hi 

It took me six months to be able to do It that much

Astro Sky Is been mewing since 5-6 years 

There was one of his video in which he show what's happening inside his mouth when he lifts the tongue... But i can't find It anymore. Maybe I'll try later

EDIT that's the video! 

Go back to 1:10 and try to replicate that at the mirror.

Follow also @drunkwithcoffee tip about push/throat swallow and chin tuck. That's important

Most likely you aren't engaging the posterior third enough or at all.  Do a "throat swallow," meaning when you swallow it feels like your tongue is making contact with your throat.  That's where your posterior third should be making contact with your palate all the time.  If you can hit that spot, your chin will move like in the videos you attached.

It'll take time to build up that strength.  Chin tuck + throat swallow as many times as you can a day to build up that strength.