With my experiences teaching English for around 10 years now, I have always had students who found it difficult to pronounce the "s" and "sh" sounds in the English language. The key is always constant practice and repetition of verbally pronouncing words through reading, even short phrases or sentences.
There is no shortcut. So, it is best to start slow, and gradually go faster each day. Never rush.
Try this simple "s" and "sh" practice and I hope it helps you overcome this pronunciation hurdle!
If you have requests about pronunciation, dictation, or anything you'd lie to practice in English, plase feel free to leave a comment on Lifelong English. ;)