Of the two given, I would go with the first (better understand personal development). It is easily understood and a natural word order (you could stick "how to" in front of it and it would read perfectly well).
The second (better understanding personal development) has issues:
- While correct, it wouldn't be my preferred word order, at least (I'd say "understanding personal development better").
- It is too easily read as "better understanding of personal development", in which case the lack of "of" makes it look like a mistake.
That said, I would suggest simply understanding personal development. You're right, we can't completely understand it, but using the continual aspect ("understanding") takes care of that, in my mind.
And merely personal development might be even better, unless you want to have several categories about personal development, of which "understanding" is just one.