We can only speculate and let you have our own experiences but I'd say that if you come quick enough with your ex. then there's nothing physiologically wrong with you. The partner with whom you can't come knows (even if she knows nothing about you still fucking your ex.) that your mind is not with her. It will be affecting her performance. I'll turn this idea around and give you an example:
My girlfriend and I were living together. I adored her, and she me. At first the sex was great, but my old nemesis by the name of Premature Ejaculation came back to haunt me. Do you know, I had a horrible time. My self esteem slipped to an all time low. Eventually, when she came home late and didn't want sex with me, one time too many, I teased the truth out of her. She was sleeping with another guy. It transpired that she had NEVER been faithful to me. All the time I was fucking her she was thinking of these other guys and how it would break me up if I knew. With that shit going on in her head, it's no wonder she couldn't really relax and get properly into what WE were doing. Her distractedness was undermining my confidence. I was too eager to please her. It might have caused another guy to suffer impotence, but in my case I just came too soon every time, and just felt generally wretched. We separated.
But back to you:
If the girl's confidence is undermined because you are destracted because you're fucking (and/or are in love with) someone else, then I don't wonder that you don't get the same sensations as you do with your ex. Her anxiety will be preventing her from becoming fully turned on. It will be preventing her pussy from getting jiggy with your cock.
Equally, now that you have a less complicated relationship with your ex. You and she will be having great sex. I know, because (daft as it may seem) I had a second time round relationship with that girlfriend (above). 6 months of the best sex either of us had ever had, then I felt something was wrong. That distractedness was back. I got a tight feeling in my guts and asked her if she was sleeping around again. She couldn't deny it.
Just decide which girl you want and stop breaking hearts.