I get that you are angry, and I do understand why. No one likes to feel deceived.
That said shut down the emotions for a minute and ask yourself if you logically think it is viable over the long term for you to keep seeing your gf while your wife is not allowed the same freedoms? First of all, while you have your gf, it gives you the luxury of remaining angry. And I do mean luxury. Seriously, this happened 6 months ago and it sounds like emotionally you have barely moved past day 1. Why is that?
Secondly, the inequity of the situation is not conducive to forming a healthy, loving relationship. You may feel justified. That said, while trust must be re-earned, it doesn't sound like you are doing much to resolve your emotions on the issue. I think you know deep down that the way you are treating your wife is not helping to heal the rift between you, but you are not willing to do the hard thing of either letting your gf go to concentrate on your marriage or the equally hard thing of forgiving her and allowing her her loves.
As for the guy being your cousin, how is that relevant? I see no importance in that.