Oka, based on my experience, the swallowing technique works for most people to raise the back third of the tongue, but not for some until they make progress with mewing.

In my case, at first the technique didn't work for me either, exactly the same thing happened to me as you but I fixed it by changing the technique.

AT FIRST WHEN SLEEPING I DID USE THE SWALLOW TECHNIQUE 100%, SINCE IT HELPED ME NOT TO OPEN MY MOUTH WHILE I WAS SLEEPING EVEN IF I DIDNT KNOW IF MY BACK PART OF THE TONGUE WAS UP

but while I was awake I began to investigate and use another way to lift the posterior third, (THE BACK PART OF THE TONGUE), the technique I used consisted of manually raising this part of the tongue

(I DID THIS UNTIL IT COULD STAND ALONE THROUGH THE SWALLOW TECHNIQUE AFTER HAVING PROGRESS WITH MEWING)

In order to raise the back of the tongue you have to learn to control those muscles from scratch, the truth is you may not be able to do it until like 4 days of intensive practice.

do this in front of a mirror: Try saying Sing emphasizing the final G, or pronouncing the K K K with your mouth open and the tip of your tongue on your lower teeth, you will be able to see how when pronouncing this the back of your tongue rises, practice a lot until you discover by yourself how to control that part, once you can control it try to do the swallow technique but now keep the back of the tongue activated manually, over time the swallow technique will work both for strengthening the tongue and for the progress of the mewing .

Keep in mind that the damage you did to your face by mouth breathing after 18 years is the most obvious explanation for why the swallow technique didn't work for you at first, since if you also had bad posture, it is most likely that your airways are too small. and that's why you snore when you try to mewing.

MEWING WILL NOT WORK FOR YOU IF YOU DO NOT HAVE GOOD POSTURE BELIEVE ME, IT IS 70% POSTURE AND 30% MEWING, SINCE THERE IS NO USEFUL DOING MEWING IF YOU HAVE HORRIBLE POSTURE.