Same story here. I've always tested as INTJ due to poor Se in the last few years. In a way, life's circumstances encouraged the use of Ti-Ni in the form of pure maths and post-graduate education in engineering. However, it wasn't until the stresses of courses finally finished that I was able to put Se back to healthy use (working out, enjoying new foods, reading for pleasure, painting). I'd say that PerC allowed me to see this side of myself over a span of a year that I spent here.
Do I regret that phase in my life? Not at all as it broadened my understanding of both how I function as well as that of other minds. Some other points that I've experienced: Undifferentiated Ni is pessimistic as it's fed through an unconscious Fe. When its out of wack, Fe projects the sub-conscious upto the Ni which presents itself in only negative value judgments of some quality / essence of something (your future, aspirations, direction in life). Ti-Ni with differentiated Ni is like a motor-arm with a swiss army knife attached to its stub. Ni lets you see the problem from all angles, Ti completes the proof. The crux is in how to give Ni material to work with. I found Se visuals were the saving grace which led me to relearn much of the maths in terms of visual analogies / models that my Ni can draw from in the future.