сел is [s'el] съел [sjel], so no any break but an entire consonant [j] (like "y" in yes), ' signifies palatalization. Ш is close to voiceless English R, Щ is pronounced with the middle of the toungue raised.
These are all questions of Russian graphics. It is like [ke] is spelt que in Spanish while [ka] is spelt ca.
As far as I can see such things cause unbelievable difficulties among learners of Russian, I don't know why.
Ш and Щ are as distinct as man and men are in English (Russians don't usually hear the difference).