Why do I still think about ex when masturbating?

Ok, I think this question is a lot more complex than it sounds..

I've been married 17 year, maybe 15 year ago I got talking to a different girl using msn. We had a lot more in common than I did with my wife including our love of a favourite sports team.. We would chat for hours and have webcam sex daily, we eventually met and had about a 6 month affair before my wife fell pregnant and I stopped the affair. I was in love with 2 women at the same time and decided for my unborn kid I had to stay with my wife. The other girl was devastated and to this day I know I can just give her a message and she would want me round her house straight-away for no-strings sex.

My wife now suffers from a condition that makes sex painful, she thinks this was caused during childbirth and now we have 3 kids - finding time or when she wants to have sex is pretty rare. It often feels like rejection when I try to initiate sex and get the 'not in the mood' excuse.
I've learnt to be sympathetic of this condition and supported her knowing there are more ways to enjoy yourselves than penetrative sex but its hard work when I seem to be the only one trying. She even has a pain in her jaw when she opens it too wide which makes oral a rare thing as well.
I know my wife does love me and would do anything for me (that doesn't include anything sexual)...but it feels like how a sister would love a brother....

Its a good job my wife doesn't mind me watching porn or I don't know how I could cope with the lack of action.

I've been tempted to get in contact with the other girl many times during the past for '1 more time' but I find after watching porn and fantasising its me and this other girl in the movie my feelings towards her go back to normal and I thank God I haven't messaged her while in this erotic state.
9 out of 10 times while masturbating I find myself still thinking back 15 years ago to this other girl and what a great sex life I could have had.

I was doing this constantly when I first stopped seeing her and thought id have got bored of it by now, but its been 15 years and I'm still thinking of her riding/sucking me everyday.

Did I pick the wrong girl to stay with?
Why cant I get her out my head?
Is it even wrong to have these thoughts?