Before you read on below …
I am going to assume you’ve read this and this. Otherwise the below will lack context 
INFJ
This is my personality type so it’s one I know well. And the whole spark for this series of posts came from me doing a Myers-Briggs course and hearing the presenter say:
INFJs are motivated to help people bring order to their lives.
INFJs are life’s advisors and can’t help but help. It really bothers them if there isn’t closure on an ongoing problem.
How do I help people bring order to their lives? By giving tips for living a less frantic life here on my blog and also by mentoring people. I love mentoring. I love the way it allows me to help someone take all the jumbled thoughts and questions in their head … and wrangle them into line. It’s awesome. Both my blog and the mentoring I do give me a very deep sense of satisfaction and put a smile on my face every day.
Carly’s mum is an INFJ. She has her PhD in Psychology and actually taught psychology for a while but found she preferred being a career counsellor (helping people find a career that fit with their personality type). She also gives out unsolicited advice regularly to her close friends and family related to bringing order to their life. If someone she knows has an ongoing problem (things like career choice, parenting issues or marriage issues), she will keep noodling at it until she gets to the bottom of what should be done. She is hugely driven to make sure people’s issues are ‘sorted out’ (I am exactly the same!).
So if you are an INFJ, you have a deep need to help people ‘sort their shit out’. (Sorry – I can’t think of a more eloquent way to put it.) If you don’t have this in your life right now, or if you do have it but it’s not on your own terms … you are probably going to have a slightly nagging feeling of discontent.
INTJ
Of all the 16 types, INTJs have the highest need for continuous learning.
INTJs thrive when they are progressively working towards higher and higher levels of competency and excellence. They are driven to improve both themselves AND the world around them.
My friend Bernadette Jiwa is a classic example of this. She is considered a global thought leader on the topic of brand strategy and storytelling. She was named one of the top 100 Branding Experts to follow on Twitter. She worked with Seth Godin (the world’s greatest marketer) on the Kickstarter project for his most recent book. But is Bernadette resting on her laurels and enjoying the rewards of all the hard work that’s gone before her? No way – she is continually pushing herself to improve further. Not just that, she does the same for every person she comes into contact with. If you sit down to have a coffee with Bernadette you can expect her to challenge the limits you have imposed on yourself and you will walk away inspired and determined to take yourself to the next level – no matter what it is you do.
So if you’re an INTJ and you’re feeling a bit dissatisfied with life – it’s probably because you have let yourself get stuck in a rut where you are not ‘improving’ yourself and/or you are not inspiring/pushing others to improve themselves.
ISTJ
ISTJs are naturally motivated to go ‘deep into the weeds’.
They like to get their hands on the nitty gritty details and logistics and take a messy process and make it tidy, smooth and step-by-step efficient.
ISTJs find a lot of satisfaction in things that involve precision and are highly concrete in nature.
Carly recalls a high level ISTJ at a retail company telling her that he maintained a second full time job at his company for many months without extra pay (when someone left the position suddenly). He was drawn towards it because it involved streamlining the process of getting products out in the right amounts to the right stores in the right time frame.
My husband is an ISTJ and that probably explains why, when he was a teacher, he ended up teaching maths despite his training being in Phys-Ed. It’s also why, when he took over running my business, the most fun bit of it for him was drilling down into the numbers of the business, finding out where things were operating in an unviable way and creating systems to make things viable again.
So if you’re an ISTJ looking for your thing – the scope is endless! I would think you are best served finding something you have an interest in first (for Carly’s friend it was the business he was working in, for my husband it was finally getting his hands on my business!) and then – get down and dirty with the details and make things right!
INTP
INTPs are the most intrinsically motivated of the 16 types.
They thrive when working for long stretches of uninterrupted time on the most complex logic problems that no one else can solve.
Carly has a friend who is a computer programmer working at a software company. At every company he’s worked, he tells them ‘send me the bugs/issues that no one else can figure out’ and that is what he works on all day. Some problems are so complex they take weeks to sort out. Several times projects he’s worked on for a year or more have been cancelled and never made it to market. For most people that is devastating to motivation. He doesn’t mind much because he (like most INTPs) is not about the product. He enjoys solving complex logic problems almost as sport.
My friend Alexx Stuart is the same. In every industry she’s worked in and every job she’s had, she’s the one that finds solutions for the problems others don’t know how to fix. Nowadays Alexx’s passion is real food and she has found a typically Alexx solution to helping people ‘flick their switch’ and want to live a life sans ‘packets of weirdness’, white sugar and flour. She has removed the element of deprivation that most people feel when they make the decision to eat ‘properly’ by creating recipes for tasty things (like Milo and sticky date pudding!) out of real, nourishing ingredients. She spends hours of time experimenting with different ingredients to find the happy balance between tastiness and nourishment and is actually about to launch a book the features 68 of these amazing recipes!
If you are an INTP you’re probably already doing your thing, you just don’t realise it. But I know from experience that having it pointed out to you is very powerful indeed.