Answer: My guess is that you are very good at what you do and are a hard-worker so they want your input and your “good work” to get things done. ISTJs’ organizational skills and efficiency at tackling work tasks are much needed in most workplaces. Their ability to do the work they are asked to do and their work values can be to their detriment because workaholism can be a byproduct.
The antidote to this issue is to say “no” to work you think should be delegated to someone else or to some other department because it is not in your perceived job description. If you have been at your job long enough to prove your worth, you are not at risk to be fired. It sounds like you are caught in the common practice in companies to reward good work with more work. ISTJs are very good at doing the work because they can, but risk becoming workaholics because their Fi function may not tell them how much is enough.