Hi Katie,
Welcome to PC. I'm glad you found us!
Let me ask you some questions here. First off, what is your "gut" instinct about your husband cheating or at the very least flirting more than would be acceptable to you? Are you hearing some sirens go off? Is it tightening when you think of this happening? If the answer is yes to these questions....go with your gut and investigate what's happening here.
From what you have posted, it sounds as if your husband really doesn't care how you feel about his behavior. That's very unfortunate. I think that any time a spouse invalidates how their mate feels, it's a huge problem (when done over and over again). It tells me they only think of themselves, they only want to make themselves feel validated, and then it's at the spouses expense.....not right in my book.
I think at this point it's important for the two of you to do some real heart to heart communicating. It must be done when both are in a good mood and neither one should be accusing the other of any difficulties. Use "I" statements.... such as, "I feel dismissed, when I try to tell you what I think of something and the only feedback I get is, "you're being ridiculous". If you both have a hard time with this, then I strongly suggest some couple's counseling. Sometimes we have to learn how to communicate with our loved ones, especially after being married for 19 years.....many things change in our lives, as a couple and individually and we need to learn new skills.
Good thoughts going out to you both!

sabby