This is a tough question. I would make sure before you approach her. Sometimes, women join gyms to better themselves for you and sometimes it could be a surprise she is working on for your upcoming birthday or anniversary. Then again, there is a chance your gut feelings are right and she is cheating on you. Women cheat for different reasons than men.
Personally, I would at dinner talk about how much you love her and how much she means to you. I would also add a story of a friend who is cheating on a spouse and how it makes you feel. Slide in an "I wouldn't do that to you" and "I hope you would never do that to me." Also, let her know if you ever did or if she ever cheated the best thing to do would be tell you/her and come clean. Yes, it would hurt but it would be less painful than everynoe else knowing it was going on and no one saying anything or you finding out on your own somehow. If she is smart she will do one of the following fess up over the next few days or she will end her affair.
After a week, if you still think she is seeing someone else. Come right out and ask her. Give her her one chance to come clean now without you flipping out on her. Tell something is wrong and you feel her slipping away from you. You have even thought she might be having an affair. Tell her if she is to please tell you now and you promise not to flip out. (Please dont flip out if she tells you she is! Just try to remain calm as you can. This will get alot more complicated as the days go on if so and there will be other times to flip out!) listen to everything she says. If she asks you why you think this bring up little things like how your sex life has changed, how she is not talking to you as much about her day, or about how much time she is spending anywhere but at home with you. Hopefully this will get her to confess. If not she might not be cheating but doing something else maybe even seeing a professional for therapy.
If you still can't shake the feeling after all this and things are not changing. It might be expensive but its the only way you will know. get a private investigater. Do NOT use a decoy to set her up since that to me is deceit at its finest. Use them to follow her 24/7 for how ever long they recommend. They will track her professionally. Keeping logs about where she goes and who she sees plus photographic evidence. Pictures won't lie. Plus, they know what to look for as far as if he is really more than a close friend or lover. This can also help you if you end up getting a divorce. Let them do their jobs and listen to whatever it might be they say good or bad.
If she is cheating on you as them if they have an advice on how to handle confronting her. They should be able to help you with the confrontation.
When you do confront her please, think about everything before you do so. Think about what you want her to do, get out, stop it, or forgive her and get professional help for your marriage. I recommend this no matter what if you are going to save your marriage! It will help you to see why she went to someone else and what you both see as problems in your marraige and how to fix them. Have your plan and feeling in order.
One final thing, make sure you stick to your guns. The hurt and anger will wear off with in time. How much no one knows. You said one thing that I feel is very important for you to tell her, You can't live like this and you don't want to lose her! Make her know what she means to you and how much you love her also how much this hurts you as well. I wish you luck with this and I hope you will get the answers you want good or bad.
