Considering the fact that you’ve never tried to hold that muscle contracted before doing kegels, it makes perfect sense that you can’t hold it for very long. As with any muscle, if you’re exercising it for the first time it is going to be difficult to control and exercise effectively but it is going to get stronger quickly.
I doubt many people can hold a kegel for 5 seconds after just starting them. And those that say they can are probably just flexing and holding the wrong muscles anyways.
Also the fact that you cane hold your pee doesn’t mean much. You are essentially doing a kegel when you stop the stream but that feeling of it backing up and easing off after a few seconds of holding it is other muscles taking over. The muscles triggered during ejaculation (and kegeling) aren’t the ones keeping you from wetting your pants during a long car ride. That should make perfect sense by itself but it is further evidenced by the fact that you can be holding a full bladder and THEN perform a kegel.
It’s probably not want you want to hear but just keep at it. Your ability to focus on and strengthen the right muscles will improve quickly.