For instance, "sun" is called "ήλιος" but the way I was pronouncing it sounded like "ήγιος", which does not exist as a word, but sounded more like "υγιής" in a very wrong way.
My mother got petrified when she realized how I was using such words (be it ήλιος, ελιές, μαλλιά, Στέλιος and so forth), because my grandfather (her father-in-law) is a mumbler - lisper - stammerer...so hard to understand him.
So, she had to act quickly: she dedicated hours on me showing me in very slow visual motion the mouth / tongue muscle movements I should use and in what way so I could pronounce the words properly.
The approach kind of resembles to the procedure of how people learn to observe the mouth in Sign Language in order to understand what you are saying.
Her trick was to find words that either had a common beginning or end and pay extra emphasis on those to pronounce them correctly and then blend those two successful pronounciation on matching "problematic, lispy" words.
In shorter than 2 months, I was able to speak normally without a problem at all.
Maybe this approach could help a bit?
You have nothing to lose; give it a try.
Good luck!
I would try contacting local speech therapists directly and see if they offer or would try online sessions.
Maybe you have already read these blog posts on lateral lisps, but they seem useful with try-at-home programs:
* https://www.grahamspeechtherapy.com/blog/the-secret-to-corre...
* https://www.speechandlanguagekids.com/speech-therapy-frontal...
Thanks! I will read these articles.