Complex sentence: At the time of his thirteenth regeneration, the Doctor discarded the bowtie in acceptance of his coming change; the Twelfth Doctor would look back on this obsessiveness, calling it embarrassing.
Split into simpler sentences: At the time of his thirteenth regeneration, the Doctor discarded the bowtie in acceptance of his coming change. The Twelfth Doctor would not look back fondly on this obsessiveness, calling it embarrassing.