Sorry Moose, I was going to respond to this yesterday, but decided to leave it 'til I had time to do it properly!
It could be argued that low self-esteem comes under the bracket of tarbiyya. I can actually also see it having a relation to egotism because your study of the ego can address questions around self-worth, where we draw it from and whether low self-esteem is giving other people's opinions a misplaced emphasis in your life? When it comes to "root causes" of low self-esteem, I seem to be surrounded by people who reference the way they were made to feel by family (particularly parents) growing up. Their idea of who they are in the world and having validation for being an important and valuable human being can be greatly reinforced or hugely stunted by negative comments and attitudes in childhood- arguably the first home of tarbiyya for human beings.
I think the tarbiyya should come with a human guide/spiritual teacher. Knowing you well enough will help them prize out exactly where the cause of a person's issues are- the hope being they can be given a tailored "spiritual prescription" for their issues. But like you say, low self confidence is just one example, I can think of sooo many other problems relating to the self (nafs) which could be mistaken for egotism. However, I can also see if you peel back the implications of these problems- they are traced back to the ego. I say this because a carefully trained and self-aware ego would be able to identify and work on these inner issues with the help of a teacher.
Yes, it depends really where you draw the circles between psychology, spirituality, self, etc. Tasawwuf, at it's core, emphasizes the integrated and holistic nature of the human being- where all faculties should be working in harmony towards a united purpose to produce a person at peace. I feel the calibre of the teachers is something which is a huge factor in the success and failure of the "spiritual wayfaring" of a student.
I think there's a two-fold almost dichotomy when it comes to the ills of the nafs. I feel like as time progresses we just have different (and often alarmingly extreme) manifestations of the same spiritual maladies. It seems a regular exasperated claim to think "we're at a unique juncture in history", "we have never been confronted with the kinds of problems we see today" or even basic claims like "the challenges of Muslims in the West is something never seen before". A cursory look at history will show that the same and often worse situations have played out and affected Muslims again and again throughout history. Even things like social media, may be unique in that we have never been able to communicate like this before, but the risks to the nafs (and everything else) that this medium affords is traced back to the same primal spiritual diseases which have always existed. I feel personally what is missing more than "bridging the gap" between spiritual thinkers of yesteryear and modern psychology is for people to have a clear understanding of the base nature of our nafs and the components which make us human. Then an idea of how to gain mastery over these things. And like my most favourite quote from Dr Zhivago, to be helped (by a human teacher) "to call each thing by it's right name and put each thing in it's right place". On a slightly side point, I'm not sure if you've read (the translation of) Talbis Iblis - Deceptions of the Devil- by Ibn Jawzi? I find it sooo interesting because it is a classic text where he is narrating and then critiquing many of the Sufi practices he sees around him in his own age. And he spares no punches. For example, he writes in one section about the routine practice of the Sufis of his area to starve themselves of provision, but to cook really delicious-smelling food and leave it where people outside could see and smell it so they would think that the Sufis themselves were well-fed. The aim of this was apparently to rid the nafs of riyaa and to purify the person who is voluntarily hungry from being praised for his ascetism. Ibn Jawzi says however, that not only is this practice antithetical to the Sunnah and an extremism- but also, it doesn't actually aid them in their fight against riyaa....since they KNOW they are making others believe they are well fed when they aren't, and this knowledge creeps into their ego and amplifies their self-appraisal as people who have fought arrogance. It was an interesting read.