@sparky779j,
If I'm not mistaken it's not about your sex ability as much as it is about her preference (so don't be so hard on yourself), Some women who were sexually active before marriage tend to favor a specific way of doing it and it's obviously not the way you do it, she had already cheated on you, and would most probably do it again in order to relive "the experience".
I recon you should do something about her being a cheater (because I believe that's something you should react hard on), I'm sorry if it sounds offensive but no matter how hard she loves you, she's favoring a penis over you.