Do not Worry!
Stammering is neither a disease nor any physical disorder, it is purely a psychological and bad habitual problem.So, We can overcome by continuous practice
There is nothing wrong with the tongue, throat or any other speech organs. When you speak alone or sing, you do not stammer, sometimes you talk very fluently. It means that it is purely a bad habitual and psychological problem. Any bad habit can be cured with a right approach to tackle the problem.