If it's something in the tire, you might not necessarily feel it by rubbing the inside of it. It could be something like a tiny sliver of glass stuck in the casing. Look on the outside of the tire too. Sometimes, a tube itself can be defective, but I mean, if it's happening repeatedly, there must be something there. Other than that, where are you riding? The side of roads and urban streets is usually laden with debris and invisibly-small shards of glass. Ride there and you're sure to get flats.

Of course, at some point you just give up and buy a new tire, preferably one with a flat-resistant Kevlar belt or something. It doesn't have to be an expensive one either. I have a made-to-measure bike with a hand-made lugged steel frame and all Italian components, but I'm quite happy to ride affordable IRC Road Winner Duro II tires on it. A few years ago, I got tired of repairing and replacing expensive racing tires so often. But before giving up, maybe you should have a bike shop look at it.