I think what makes most people start doubting their gung fu is that they just take too big a bite and simply can not chew it up. Most systems of gung fu require you be in great physical condition to excell. And until you are, you will not go very far. There is no secrets involved that will make you a fighting master. You have to work very hard. And the first step is to learn the principals of your chosen system and learn them step at a time. Develop your fighting skills a step at a time too. Learn them well and complete before moving on to the next systems of techniques. Even shadow boxing will develop your fighting skills. I also think the thing to do would be to look into the different skills and match them up with your own physical abilities and expectations. If you are not going to ever be capable of achieving a really high level of physical fitness they you are simply ****ing into the wind.
When I was quite young and in school, I knew individuals that could not even spell words like gung fu or karate and probably never even heard of it before, but they had a few fighting skills that they had trained hard and could apply smoothly. Their skills were honed by application. They were tough guys and had reputations for being skilled fighters. These skills were actually limited to a half dozen moves that were well polished.

Chiang