Hello Muhammad.  Happy New Year.  You are quite right, there is nobody like your Mum.  Everyone's Mum is absolutely special to them and rightly so.  Makes no difference whether you are a boy or a girl; this is your Mum and she is Number 1.  You don't say anything about her being ill at the moment (is this correct?) so I am guessing that this is just general fear of losing her?  Is your father around?  Do you have brothers and/or sisters? 

While it is true that nobody is guaranteed a long life - we just don't know - it appears that you have nothing to worry about at the moment regarding your mum's health.  But you are worrying.    Have you lost any other family members at an early age and is this why you are worrying?  Apart from this worry are you okay generally with what is happening with your life?  I have found it generally helps to have friends and/or relatives with whom you can share your fears.  Or perhaps if there is a mosque you attend with someone there who will talk to you about your fear?  Fears are always so much worse when you bottle them up within yourself (believe me, I know).  I am not a member of the Islam faith so looked up the early life of the prophet and I see he lost other family members, his father before he was born.  There is no reason to think you will have to face the same heartache.    I hope I am not saying anything offensive or the wrong thing but although I don't know your exact age I think this is a time in your life when you should be learning and enjoying new experiences without being upset by these fears.  My best wishes to you.