dear jane, you know it's wrong... so i guess since you've
started, and your son is at that age, you gotta do the difficult
thing now and to have a heart-to-heart talk with him...
and then to make a decision not to do so again and stick to
it...
maybe your desire stems from being with him for extended periods of
time without meeting other people (please correct me if i'm
wrong)... so what you can do is to go out and meet people, get to
know them...
who knows? you may find someone for yourself 
whatever it is, you already know deep inside you this is not
right... and therefore you must stop.. that is the most important
first step which you must take... after that, you have to steel
yourself and not let your desires get the better of you...
good luck..
like i say to many.. we all get a little lost sometimes...
