I am going to speak only from personal experience, from recent events in the life of some very close friends and from generally accepted concepts. Small references to scientific points of view may appear as an example but they are not to be taken as a direct evaluation of your situation.
No relation is ended without leaving marks and no sex is completely without emotion (unless the person in question truly is a psycopath). Even in relations as strange as the ones which appear from prostituition there is usually some emotion attached, on both sides, for each other and, as paradoxical as it may be, some of these emotions may be good.
Think of the Stockholm Syndrome: people captured by criminals, or even terrorists, will sometimes feel empathy for their captors and go as far as to defend them at some point, even if they are in real danger, discomfort and fear. This is an instinctive defensive posture that can lead to persistant feelings of abnormal compassion towards those who did, or would do, harm to them.
These are extreme situations but they make it clear that bad or unwanted relationships can leave a paradoxical good feeling behind.
You said you were codependent: that is a very bad position to be in a relationship. In a way, you may have felt a skewed pleasure during the relationship you were going through, and I certainly don't mean just the sex, but you must realise that.
A close friend of mine went through a similar situation, although with someone who doesn't fit the profile of a psycopath, as far as my limited knowledge allows me to conclude. At first he was lured out of a very healthy relationship mainly by lust and the promise of intense love. When things started getting ugly he realised something was wrong but, even though some people advised him of the girl's true intentions, he kept investing emotionally in the relationship.
At one point he tried to end the relationship but he later confessed something inside him made him feel sorry for the girl, as she pleaded for him not to leave her, and he turned back on his decision. Finally, because we are like brothers, after he told me thourougly, many times over, how deeply distressed he was and how much he was suffering, I took him to the edge of total misery, forcing him to truly see what he was getting himself into. It was one of the hardest things I ever did. As soon as he truly felt how disfuntional his relationship was, I immediately encouraged him and did everything I could to make him feel as well as he usually does, sacrificing some of my own privacy to give him reasons to feel better. This led him to decide to break up with her at a startegically chosen point in time and he went through with it. He felt a new man afterwards.
I don't now if my young friend still thinks about his ex-girlfriend, not enough time has passed to evaluate the situation, but I will be trying to find out so I can help him avoid your situation. He is not codependant by nature, not in the least, although he behaved as such during some phases of his unfortunate relationship, but it is still possible he will suffer from an “abstinence syndrome” all the same. If you are truly codependent and the sex was that good for you, it is only natural you should think of her with longing, and here's why: now that you are no longer under her negative influence, although the reasons you left her are still in your conscious mind, you probably no longer “feel” them as you did before and only admit, at the deepest level, the positive sides. You “know” the relationship was negative for you, but you don't “feel” that way anymore.
For your sake and, most importantly, for the sake of your children, you must try to be assertive about this and reason your way into the deep belief that you cannot return to such a situation. Even if you don't bring her back into your life (which I hope wasn't what you were thinking of doing when you came here) keeping that obcession is going to result in three things: constant suffering, bad social relationships (for example, with your children, which is a terrible thing) and a profound difficulty to move on with your life and try to find a truly balanced and loving relationship with someone else. And there is always the risk you will end up in a similar relationship just for want of something equal to what you had before: abused women sometimes seek abusive partners when the original abusive relationship is over, if they are not helped, that is (although it is not for the same exact reasons you might do the same thing).
As I always do in these posts, I want you to be completely aware that I have no real authority to support my point of view in some of these matters, only my experience, and that is subject to my own relative, and fallible, point of view. Another thing I keep saying to everyone here is: when in doubt, find a therapist. You may be able to overcome this by yourself, but if these feelings persist you need to get a professional opinion. A therapist won't have a miracle solution, but talking to one will start helping you get a little perspective and can, at least, make you feel better quickly, even if only a little at first (which is certainly better than nothing).
If for nothing else, go to a therapist to get help for your codependence: it is a terrible way to live a relationship and, although technically they are benefiting from this behaviour, healthy women who are searching for an equal companion, and not a pet, will probably reject you once they realise this is how you are.
Meditate on this and plan carefully your next course of action. Your emotional life, and that of your children, may depend on this.
"Courage is resistance to fear, mastery of fear - not absence of fear." - Samuel Clemens (a.k.a Mark Twain)
-- What am I doing here? --
I am currently doing research on several topics for a few books and articles I am writing. I would like to make a positive contribution to fora (plural of "forum") about such topics in order to get constructive views on those matters for a better perspective on them. This is why I came here. I will not indulge myself in flamming and will not respond directly to any clearly non-constructive replies.
I have read some of Dr. Robert's responses to certain questions and I find them lacking and biased. Still, I truly wish to know what people who come here, and even Dr. Robert himself, think about my points of view for three reasons. First: I know I can, very easily, be wrong and I should keep an eye out for other opinions. Second: all points of view are important to have perspective on any matter, specially the ones one doesn't agree with. Third: wisdom can come from very strange places and one should not underestimate anyone as far as knowledge and wisdom are concerned.
Please, give me honest and constructive replies: I will return in kind.
"The slumber of reason creates monsters." - Goya