I would not consider missing your ex as cheating on your spouse. I would, however, consider it cheating if you were acting on this feeling, such as calling, texting, emailing, or seeing your ex. If this feeling is overwhelming, you should probably try to find out what is lacking in your current marriage that is making you feel your spouse isn't as satisfying to you as your ex. And think about the reasons your ex is your ex and not your current partner. Then, I would suggest working on your marriage so that it makes you as happy as the memories of your ex. If you can't find happiness with your current spouse, and continue to miss your ex, then I would suggest seeking professional counseling to help you sort through your feelings and take the next step.