My wife is chatting with someone which she hides from me !

Trust is one of the most necessary foundations of any relationship. I see that it is now becoming questionable for you in your marriage. Your wife chose to keep her chat from you until you found out about it 2 years later. What that chat means and what about it bothers you is something for you to think about... it is her hiding it from you or her chatting with someone or anything else?

Why someone chooses to keep something from someone is that either they are doing something that won't be liked or that they won't be approved or understood or accepted because of it. Try to explore what exactly this chat was about and what about it made her keep it from you and lie to you about it. Clearly you seem to want to understand why she would choose to do something like this knowing that you would not like it, which means that you do know her enough to expect her to not do something that would hurt you. It would help to also try and explore if there is anything about your marriage that was, for her, not in place which made her take the step in the first place. It would help to start building on the trust and communication in your marriage by finding out what went amiss and how best you both could mutually work out together.

Going for couple's therapy would help you both find a safe space to explore your issues and resolve them together.