Great poem. Beautiful message, so painful. There were some flow problems, it didn't read too well, but thats easily fixed my suggestions:

Mum where did you go
Without you by my side
Im am so lost and confused

My heart is heavy
And my pain is great
And i miss you everyday

Please come back to me
My life has no meaning
I have nothing left

I am an orphan
That word and its meaning
Is too hard to hear

I dont know where to turn
I dont have strength to fight
I wish I could be with you

Maybe i will soon
As the longing inside me grows
It will surely get worse

This sickness that I hide
Will soon be too hard
So i just wait for the day
I can run into your arms

Having sickness twice so close together weakens the strength of the final stanza, sometimes switching a line around fixes flow problems and gives the poem more impact. I hope you like and use my changes. :) Your poem is so expressive of real, raw pain, like I said earlier, beautiful, it just had a few minor problems in writing, nothing wrong with what you were trying to say.

This is so sad dear :( *hugs* But nonetheless, a good piece of poetry. I hope life would make a better turn for you (: