First you have to figure out If you really love your wife or not, may be you don't want to leave her out of fear of family, society or fear that she will be heart broken. If you love your wife, you should have left your mistress already but since you can't help , it only points that you have no love or commitment for your wife.
what can you do now?
1)if you love your wife or you have figured out you really do love her: the best thing is to come clean, it is hard for you both but trust me that is the most healthy way in the long run if you guys want to carry on.Tell her you made a mistake with another women and if she wants to forgive you, you will never do it again. Let her make the decision , you just be honest to her.
if you have decided to tell her: Few things to keep in mind :
i. Always stay by her side until she recovers , which she will with the time.
ii. Take care of all the house work.
iii. If she ask you something related to affair: give her honest answer but don't give her detail, she will over think.
iv.wipe her tears if she cries.Go to your religious place, temple or church together and prey as much as possible. It is good way for her to let her thought outs and have some peace.
v. Take every emotion she throws at you , she might slap you, spit on you, get mad at you, take everything without leaving her side.Just remember, it will be over one day.
2) if you don't love her but want to carry on because of fear and hide your affair : This is the worst mistake you will be doing to yourself and to her. You have to live with the fact that you are fooling her and also yourself. Life is long friend, path is very long. I HOPE YOU WON'T CHOOSE THIS PATH.
3) if You don't love her and want to divorce her : Tell her in a good way and comforting way as possible that you that you don't love her and it is in best interest for both to separate.
4) if you still won't leave your mistress ,keep cheating and keep living in guilt : Good luck living in hell, there is nothing in the world that will give you peace and bring you out of that guilt.DON'T TAKE THIS PATH,PLEASE.
5) if you don't want to tell her but will never make the same mistake : You have to live with the guilt that you have fooled her once and had an affair with someone. Your relationship might go well but your guilt wont go away until you confess to her.
This is my suggestion to you, decision if yours.