My husband & I have been married for 23 years our two children are grown . He is committing adultry & cheating with another woman I am devastated & mentally depressed & the mental anguish he has caused me is wrong . I trusted this man and now after 23 years of marriage he is cheating on me I am very sad & feel betrayed what is the best way for me to file for a divorce & should I get an attorney ?

Yes, you need an attorney. You need to start now to gather some money in your own name and do it without his knowledge, and make copies of all important papers and documents. You need to make plans and tell any of your family and closest friends that you plan a divorce (tell the children later, unless you already know they agree with you). You might want to see a therapist to help you get through this.
What country are you in, and if the US, what state? Do you and your husband own the home you are in? Do you have enough combined assets for him to easily live apart?

Agree, do you have your own income and money ? Move most of the money to a account just in your name. Hire an attorney and file for divorce.

Often the reason for divorce does not matter, they just proceed, sometimes it matters in some settlements Your attorney will know that.