Before you start on a road that you cannot turn back from, such as hiring an investigator, please consider if your relationship can be saved before deciding on this extreme.
Is the relationship that you have invested time, money and youth on no longer worth fighting for? Many people in such an ambiguous situation may not be able to make the hard decisions because of their kids. Or for many in Singapore, divorce is just too expensive an option.
The following are some questions you may ask yourself which might actually advise you what you should do next in order of seriousness.
1. Is he / she in the happiest moments of your life that you can remember?
Think about the times you dated and the good times that you both shared together. Was he / she in most of those moments in your life? If even the happiest point with each other isn't at the high end of your scale of 10 signifies that this person is unable to bring true joy to your life and may not be the right one for you.
2. Has violence occurred in the relationship?
Has he hit you or physically abused you in anyway? Today, if a friend complains to you that their boyfriend or husband is hitting them at home, normally you would advise them to drop the relationship for their sake. Why not apply this on your own relationship? Sometimes it is difficult because on the very next day he apologizes and seems sincere, but before you know it , it happens again. If violence has repeatedly occurred in your relationship, I would firmly advise that you will be happier leaving your spouse.
3. Has he/she crossed the line?
Have you clearly stated certain reasonable rules and lines that your spouse should never cross; such as something simple like never to throw away anything without your permission or ultimatums of never to cheat? Has he/she crossed this line accidentally or on purpose? If he/she disregarded your ultimatums on purpose or not, that was the last straw and you should do what you deem is appropriate in your case. If it was voluntary in breaking the rules, it may indicate that he wishes to get back at you for a disagreement that you had in the past. Or it may be seen as an outright rebellion against your wishes and a clear sign that the love is no longer there. Find out which one and use your discretion.
4. Has he/she been cheating for multiple times even after seeking your forgiveness each time?
You already know that staying is a pointless effort and that it will never change. You say you love your spouse but is it vice versa? Can you continue loving the 'cheating' side of him/her and accept it like a bad habit? Think about yourself and your future. There are much better choices and options out there, just that you might have missed it while closing an eye to your spouse's carnal actions. If you can't, think about your children's future. No sane adult would want that kind of behaviour rubbing off on the kids as they grow up under that influence. You will be much better single than having a chronic cheater by your side.
5. Has he/she already started a new life with another partner?
This includes buying a house for the other party or renting a room together on a long term basis. The worst case scenario is that another family (with children) has already been started already out there. If it really does happen, we are deeply sorry to hear about it and we advise you to fight for your rightful alimony if a separation is proceeding.