You are definitely not alone. You might be surprised at how hard it was for the couples you were with to find you. I know that when I was a single female there were lots of frustrated couples looking.
Adult Friend Finder can be a successful place to go, but it does take time and work. I met with a few couples from there as a single woman and my husband and I are there now as a couple. However, if you live in a small area then that is going to make your search for the elusive unicorn even more difficult.
A few suggestions (which you may or may not like). You can look into swinging. Finding a couple is often lots easier then finding a single female. Plus many couples do all sorts of things with others. You may even find one where the guy won't mind sitting back and watching the action. Just make sure you talk it all out with your man first.
You may also be able to find a girl in a swing club. Of course, if you are like us...the nearest one is five hours from here!
Good Luck, try to stick with it and try not to feel so frustrated. It can be a hard search. Till then you can try other things like fantasizing you are a different girl or that you are with a different girl. This too can be fun!