It seems that both of you have entered in extra marital affairs for different needs. Majority of women seek emotional support and concern from extra marital relations, while men have primarily physical interest, in such relations. Women will always remain more attached then men in such relations (exceptions are always there!) and more so if her emotional needs are satisfied. What you interpret as craziness for you, might be her emotional dependency on you. In such condition, she may react wildly to your back steps in relation and that is what exactly you are anticipating. So you need to do this gradually but surely (if you really wish to do so, otherwise there are incidences of going back and forth). Reduce your involvement in her personal matters and ultimately stop. Do not talk long and daily. Stop sending any emotional signals. She may react wildly; emotionally or in a threatening way but you need to remain steady in your approach.
I do not know why you want to call off this relation but I hope it is not because you had what you wanted and now loosing interest. If this is the case then it is emotional cheating on your part. Please remember this and do not do this to anyone.