Since you already own an airplane and you bum around the sky on the weekends, why not turn it into a learning experience? Use that time to train towards your instrument rating. As it's been pointed out, perhaps your failures (I don't tend to think of them as failures) are attributed to Flight Instructors pushing you through the system before you are ready. Unfortunately this is a common trait I found when working at the local pilot factories. A good Flight Instructor should not sign you off for your test until you are ready. As AtlCSIP mentioned, your training is about your learning, not about my paycheck. More often than not, I think that poor learning is attributed to the the Student, Flight Instructor relationship (this only applies to students who actually have an interest in learning how to fly). I have only ever had one student who had no reason being behind the controls of an airplane. In the end I fired him as my student. A good flight instructor will tell you if you don't have what it takes. And most of what it takes is a genuine interest in aviation.
Now whether or not you can obtain an airline job with said fail rate might become an issue, but aviation is much much bigger than just the airlines.