Bearing up under is a phrase that denotes strength, resilience and the ability to persevere in the face of adversity. The antonyms of this phrase would therefore be words that convey a sense of weakness, fragility or giving up.
One possible antonym for bearing up under would be "succumbing to", which suggests a surrendering to pressure, defeat or despair. Another antonym could be "yielding", which connotes a lack of resistance or the act of giving in to a challenge. Additionally, "breaking down" or "collapsing" could also be considered antonyms of bearing up under, since they refer to a total loss of strength or incapacity to endure difficult situations.