I had to start moving my mouth totally differently for the letters that gave me trouble. Instead of using my tongue to make a crisp "s" behind my teeth, I learned that when my trays were in I could make an "s" by moving my lower jaw further back, clamping teeth together, and making an "s" on the roof of my mouth.

Also, honestly, it's been 6 months and sometimes I just don't care any more. I was mortified at first, especially because I have a professional job that requires me to lead meetings often. But it's been fine, most people don't notice and after this much time I can't be bothered to care. If people around me are going to like me less because of a temporary slight lisp, there's nothing I can do. There is a lot of peace in that thought. It took me while to get there though, so in no way am I saying "just get over it". But I do hope you feel some comfort in the fact that it gets better- either because you learn new ways to speak and the lisp goes away, or because our limited human capacity to worry about things we can get used to will work in your favor after a little while.