I had this same problem when I first started figuring out my type. I thought I was INTJ at first, then INTP, then ISTP, and I finally landed on ISTJ. I had a hard time believing my J because I'm not a neat freak or anything. I rarely clean unless things start getting out of hand. The J is all about how you handle your external life, anyway. Like, for instance, one way you can tell a J or P is how someone dresses. A P is going to care about their clothing, but won't care much about having a pulled together coordinated look. A J will want everything in place in their outfit, everything to match, and will have an organized look to them.
How you handle the outside world and how you handle yourself are two different cognitive processes. With an ISTP, you approach the outside world with Extraverted Sensing, meaning you're looking to adapt yourself to the external world. You then use Introverted Thinking to maintain yourself, so you use an internal set of logical rules and take things apart in your head.
An ISTJ uses Extraverted Thinking for the outside world, meaning they deal with it impersonally and objectively. They want it to be structured and task oriented. ISTJs use Introverted Sensing for dealing with themselves, meaning they focus on having a vast memory of facts and details, without even realizing it. Introverted Sensing is about having your own set of personal experiences and expectations, also. People who use Introverted Sensing, instead of adapting themselves to the outside world like Extraverted Sensing, adapt the outside world to their own set of rules.
The cognitive processes aren't THAT reliable, though, in terms of how rigid they are in usage. The truth in their definitions is reliable. People use all 8 of them, just some more effectively than the others.