Whether it is love or lust, once you feel guilty about it you should not go further in this. Guilt is a worst feeling to live with. Check your emotions and stay away from any kind of contact with him. If you really do not want to get in to it, honestly confess your lust to your husband. Boredom can be one of the major reasons for developing such relations. I’m not sure that whether you are working or not but if you are not then start working. And finally if it’s mere lust from his side then it would get over with physical relations and you will be left with feelings of being used up.