Via a counter-claim and response to her complaint. However, I strongly suggest retaining an attorney. Allegations of fault should not be made lightly. Making such a claim brings up legal expectations of evidence. Those have far ranging consequences for the case.
You need to retain an attorney to explain and review your options.
You need to consult a NH divorce attorney. You are free to file a fault divorce and site her infidelity as grounds. If you can't afford an attorney there is some information on the State of New Hampshire website that discusses the forms and processes needed to file for divorce.
I would strongly recommend that you speak with an attorney. There are specific pleadings required for adultery claims -- which you will need to include and which you could use to get the claims against you dismissed if they were not presented properly. With regard to your claim, you will need to identify the person with whom she committed the adultery and serve them with a copy of your counter-claim and orders of notice (unless they live out of state, in which case the process is a bit different).
Disclaimer: This response is offered for educational purpose only. This response in no way creates an attorney-client relationship between Anna Goulet Zimmerman/Law Office of Manning & Zimmerman, PLLC and the recipient. Responses are general in nature and should not be relied upon as legal advice. The response given is based upon the limited facts provided, and could be different if additional facts were known.
Agreed, please consult with a divorce attorney in New Hampshire. In re: adultery, please see this FAQ about affairs/marriages in New Hampshire and "grounds for divorce." It may be helpful for your circumstances. That said, it does not supplant getting personal and confidential legal advice from a local attorney. If you have further questions, please don't hesitate to contact me (familylawnewhampshire.com).
Proving "fault" can greatly increase the cost of a divorce and seldom results in value to the parties.
Have you considered mediation now.
The court will usually order you to mediate.
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