Welcome to the forums
It is easier said than done, but don't let the frustration show in your communication with potential dates. I am not sure what you have tried, but here are a couple of ides.
1. Go with your girlfriend to a "clothes optional" resort. Even if you are not interested in swinging or recreational sex, you can meet people who are open, flirty, approachable, and who are not necessarily swingers. Also, hanging in the conversation pool for a couple hours per day over a few days may result in you making more friends that in years of online "dating."
2. Use sites like OKCupid and contact people who are "available" and either state that they are non-monogamous or have omitted the "must be married" from the "you should contact me if" field. Also, once you answer the poly-oriented questions, look for people with whom you have at least 90% match.
2.a. Location may matter when it comes to dating sites. I tried for a few months to befriend anyone on OKCupid while I was living in Manhattan - despite the seemingly large number of women, nothing materialized, not even a meet up. After I moved to Connecticut three weeks ago, I decided to try the site again. In a week I met one person and have two more dates lined up (and this is from contacting no more than 10 people in total).
3. Looks. You mentioned that you are a bit older. If you show any signs of baldness, I'd suggest shaving the scull. Whether we like it or not, it seems that shaved beats balding by large margin.
Best of luck,