Complex sentence: Almost immediately he began second guessing his design and feeling it was inadequate, a trait that he showed even when previously working under, and begin praised by, Augustus Saint-Gaudens.
Split into simpler sentences: Almost immediately he began second guessing his design and feeling it was inadequate. This negativity was a trait that he had shown many times in the past, even when previously working under, and begin praised by, Augustus Saint-Gaudens.