Lhyse, you cannot file a criminal case against the mistress alone. In the first place, it is your husband who is primarily responsible for the crimes of concubinage and repeated marital infidelity. It is because he is married that makes their act and relationship a crime. You have to file a case against them both. your husband, and you, cannot bake the cake and eat too.
for once, do not believe everything your husband says. don't buy that nothing is going on because there's someone else in the room. if you want peace of mind, you can start by accepting what is really going on. your husband is cheating. unless he changes his ways, you will never be happy. at some point, you have to put some blame on him. don't just look at the mistress.
now if you want to go after the mistress, you can file charges for damages under Article 26. remember that the mistress also has rights to file criminal charges, as well as claim for support for their child, against your husband.
lastly, i hope you won't take offense on what I said. i know that you are confuse now and text messages can be read out of context or misunderstood. but I sincerely just want to help you.