I have come to say two different kinds of Ss. The normal S when im fluent and a modified one when im blocking. The modified one sounds similar but I place the tip of my tongue deeper into the roof of my mouth and not behind my teeth. The subtle change in movement is often enough for the block to break and I can move on. People can still hear a slight difference but everybody can still understand me. The people that speak to me a lot come to understand that I do that to avoid a block. Please understand that this is an avoidance technique. It can help get out of uncomfortable or painful blocks but it wont help with actually learning how to say normal Ss as a fluent speaker. Please consider speech therapy for that, especially if you are young of age.