Blisters and sore, thickened skin of hard palate... what is this? (see picture)

[EL77]

I've had small translucent blisters on my hard palate for 12 months now, that just never go away. They only dry up a little bit (during the night), but then fill with fluid once again.
They are firm, I cannot pop them. The exact same blisters have been in the exact same spot for a year now.
The skin beneath/around is painful and swollen and has thickened. Sometimes the blisters get inflamed.

I've been to a dentist, dental surgeon, dermatologist... they don't know what this is. They are NOT canker sores/ ulcers. Could it be some kind of defect of my mucous glands, or what else? What could this be?

I tried Clobetasol ointment, but this only gave me some relief. Blisters did not go away.
My hard palate hurts every day, not just the blisters, but the thickened bubbly skin surrounding them.

See the picture (translucent blisters in the middle, around/above there's thickened bubbly skin) : https://ibb.co/7YFPRKy

Please help!

Thank you.

[EL77]

Sorry, I now realise the blisters and skin thickening are on the SOFT palate, and the junction between the soft and hard palate. I thought the soft palate was further back ;).