I m a sunni muslim follower of Imam Abu hanifa.I have done zina with the mother of my wife around 20 times'before and after my nikkah but now I stoped it completly.IS MY NIKKAH VALID?

What you did was a great and punishable sin in Islam, infidelity and unfaithfulness toward your innocent wife!
However, repentance to Allah SWT is the first thing to start with. Make an oath with Allah SWT not commit such a sin any more, neither with this lady, nor with any other woman.
 
As of the validity of your marriage contract; it is still valid according to the stronger opinion of scholars. So, do not worry about that, just focus on repairing the major damage you did in your relationship with Allah SWT.
 
May Allah SWT forgive your sin, accept your repentance, and grant you steadfastness in applying your religion and refraining from committing sins.