Participant,
Not being an expert here, it is the same as not underestimating a price of a diamond. You will either sell it too low, not polish it right; or not buy it when you could. If you do not know your own value, you will not use your talents appropriately; you will not improve them; etc.
Again, I suggest going thru works such R Twersky. I really do not have enough confidence
One practical suggestion is to get involved in some competitive endeavors, such as scrabble, chess, checkers, spelling bee, SAT, LSAT, running … – if you challenge yourself and then compare your performance with others, you will see better both your strengths and witnesses. Caveat: once, as a youngster, I challenged a much older and extremely accomplished world-renowned person on a minor issue, and he suddenly opened up with a long tirade that despite his obvious achievements, it is not just “you, young man” – “everybody always disagree with me and I have to prove it every time …”. So, maybe, self-esteemed is not fully curable by achievement – but it is worth trying.