If you don't know the sound difference between श and स it looks like you should be practicing Nepali or Sanskrit rather than English. My view on this whole accent business is it is what it is. If you are talking about pronunciation, then it's a totally different arena.

Having said that, I would like to share a funny anecdote from my college days. I used to work for the IT department and we often had to change hard drive in faculty's machines. Once I had to change a hard drive of a professor's computer. When referring to the hard drive in front of my supervisor I kept saying Hard Diks, instead of Hard Disk (Diss ka). I used the s after the k and I had been doing it for god knows how long. An American kid who was listening to the whole conversation later pointed out that I was not talking about a hard drive but multiple erections (hard Di*ks).
It might just be me, another word I used to mispronounce was ask. I often said aks. Ask is actually pronounced as "ass" then the "ka."

FF