I've only really studied it the past 3 years or so. I first encountered it some 10 years ago, but I did not go beyond taking a test for fun until 3 years ago. I think it really caught my interest when I first began testing as an INFx on occasion. Previously, I tested as INTP
always, and it never resonated with me, so I brushed the system off.
My interest started slow, then the obsession took hold of me & I began to devour as much as I could get my hands on
(as is often the case with my "interests", which I go through phases with).
This knowledge has changed me by further helping me realize that people think differently. I have always known that in a way, but knowing HOW & WHY has been useful. It highlights a lot about my dynamic with others & their dynamics with each other & so one & so forth.
More than being useful in a practical sense, it has simply been a tool to
understand people as a whole better (as opposed to real life individuals). It has also helped me re-frame my own identity. Acknowledging myself as a "feeling" type has allowed me to be comfortable with certain aspects of myself; I think this process was happening naturally, but it allowed me to clarify what it even was. Mainly, that's what MBTI has done - allowed me to clarify & articulate these aspects of people & myself that I had a sense of before but no language to describe it with.
The most difficult thing I need to overcome is probably personal bias, and whatever limits my own intelligence gives me.